In The Name of Allah, The Merciful, The Compassionate
The total number of companions, may God be pleased with all of them, is not known by an exact number. Based on al-‘Irāqī’s assessment there are more than one hundred and fifty thousand companions, though there was no official census. We know that forty thousand witnessed the Farewell Pilgrimage and seventy thousand witnessed Tabūk with the Prophet, may the peace and blessings of God be upon him. Abū ‘Abd Allāh al-Ḥakim places this at four thousand and for al-Dhahabī, the number does not seem to exceed two thousand. The fact that relatively few ḥadīth have reached us can be explained by the fact many companions were deterred from coming to the Caliph, ’Umar, may God be pleased with him, because he would interrogate them and demand corroboration. The statement of Ibn Sirīn, is instructive in signalling and reflecting a movement to ḥadīth criticism, or more accurately, character criticism:
لم يكونوا يسألون عن الإسناد فلما وقعت الفتنة قالوا : سموا لنا رجالكم ، فيُنْظَرُ إلى أهل السنة فيُؤْخذ حديثُهم ، ويُنْظَر إلى أهل البدعة فلا يؤخذ حديثهم
‘They did not ask about the isnād but when civil war (fitnā) arose, they said: name to us your men; those who belong to ahl al-Sunnah, their traditions were accepted and those who were innovators their traditions were neglected.’
The fitna here could refer to two signifiant events that happened very close to one another: the fitna between ‘Ali and Mu’awiyah, may God be pleased with both of them, only decades after the death of the Prophet, may the peace and blessings of God be upon him; and, the assassination of the caliph ‘Uthman, may God be pleased with him. It is worth noting that Ibn Sīrīn was from the generation after the Companions of the Prophet, the tabi’īn, may the peace and blessings of God be upon them all, although there were still companions, mostly junior, around during this time. Another event, though no less significant was that people were narrating without mentioning the companion – the link to the Prophet, may the peace and blessings of God be upon him. The mursal was born. It’s worth reiterating that the companions, all of them, possess the special trait that their integrity and uprightness cannot be questioned Generally, the uprightness of a narrator is judged by the external state of the person. However, the Companions inner uprightness, or their inner purity has been affirmed by the Qur’ān itself. Hence, they are above criticism. However, this does not preclude us from questioning their understanding of the ḥadīth. Since the strength of a tradition is drawn broadly from two sources: uprightness (‘adālah), and memory or retention (ḍabṭ). We do not question the former but can with the latter. We find the great Imām Mālik, may Allah be pleased with him, saying:
كَانَ مَالِكُ بْنُ أَنَسٍ يَقُولُ : لا يُؤْخَذُ الْعِلْمُ مِنْ أَرْبَعَةٍ ، وَيُؤْخَذُ مِمَّنْ سِوَاهُمْ.لا يُؤْخَذُ مِنْ سَفِيهٍ ، وَلا يُؤْخَذُ مِنْ صَاحِبِ هَوًى يَدْعُو إِلَى بِدْعَتِهِ ، وَلا مِنْ كَذَّابٍ يَكْذِبُ فِي أَحَادِيثِ النَّاسِ ، وَإِنْ كَانَ لا يُتَّهَمُ عَلَى حَدِيثِ رَسُولِ اللَّهِ صلى الله عليه وسلم ، وَلا مِنْ شَيْخٍ لَهُ فَضْلٌ وَصَلاحٌ وَعِبَادَةٌ ، إِذَا كَانَ لا يَعْرِفُ مَا يَحْمِلُ وَمَا يُحَدِّثُ بِهِ.
I do not accept knowledge from four types of people: (1) a person well-known to be foolish, even though all the other people narrate from him, (2) a person involved in committing heresy and calling others towards the innovation in religion, (3) a person who lies in regular conversation with people, even though I do not accuse him as liar in regards to ḥadīth, (4) and a person who is pious worshipper or scholar, but does not properly and correctly memorise what he narrates. (emphasis, mine)
In our circles today, people often conflate piety with knowledge: being pious does not necessarily equate to being knowledgable or reliable in passing (or even teaching) knowledge.
This is the period that witnessed the emergence of the teachers of Imām Abū Ḥanīfah and Imām Mālik, may God be pleased with them. In the early and mid second century, writing was still rudimentary, known as saḥifās, though the companions did have specific writings on certain topics, what we would refer to as jotters today. The junior successors, the tabi’īn, began composing musannafs, legal transcripts, reflecting the new challenges the Islamicate was facing, as governors, teaches and judges. They were in effect acting as independent thinkers, or mujtuhidīn, in their own right. For example, in Kufā, apart from Alī ibn Abī Ṭālib, Mughirah ibn Ṣhu’bah, and Abū Musā al-Asharī, we observe Qādī Ṣhuraih ibn al-Hārith, Alqama ibn Qays al-Nakha‘ī all of whom were very close to ibn Mas’ud, may Allah be pleased with all of them. They then became regional in their outlook, and included prophetic and non-prophetic literature, with the latter being greater in number. 
Forms of expressions and tadlīs
As transmission begins to proliferate, forms of expression become significant with tadlīs, the act of hiding a discontinuity in the chain of a report’s narrator, also known as irsāl khafī. For example, saying that, ‘so-and-so said’ implying that he had heard the narration directly when in fact he hadn’t heard it directly. Though this was not always done for insidious reasons. Al-Khatīb al-Baghdādī and ibn Ḥajar have written major books on this topic. It is also worth noting that when the classical scholars, the mutaqadimīn, were using words, for the most part, it was in the literal or linguistic sense, e.g. ḥassan, or ‘good,’ and was not a technical usage – they simply meant that it was okay as a narration.
During the late second and third centuries, we observe a shift to ḥadīth collected according to the transmitters, or isnād. Non-prophetic statement are being dropped. This is still a stage where ḥadīth are being collected irrespective of their authenticity or otherwise – they were simply acting in effect as chroniclers. The masanīd became the raw materials for the six books of ḥadīth. The condensation or filtration process was given birth. For example, we find Isḥāq bin Rahwaih, a compiler of musnad, proposing to students that “it would be better if you collect the authentic traditions.” This spurred Imām Bukhārī to compile his ṣaḥīḥ. Until now, the books were inaccessible for the layman. So in the third century, there are two things that are important to mention: this generation committed to writing all the ḥadīth that were in circulation, and secondly, ḥadīth criticism was nearing its completion. The fourth century welcomed the era of ḥadīth preservation. The demarcation between the early generation, the muta’akhirūn and the latter generation, the mutaqadimūn is chronologically set thus:
الحد الفاصل بين المتقدم والمتأخر: الحد الفاصل بين المتقدم والمتأخر هو رأس سنة ثلثمائة
The demarcation between the early generation, the muta’akhirūn and the latter generation, the mutaqadimūn, occurs at the end of the third century.
Though it is difficult to draw a line between the two generations, according to al-Dhahabī the boundary seems to occur around the end of the third century. Aside from looking at this purely from a chronological perspective, it would be helpful and more useful to view this as a change in method: the early generation was an era of direct narration of prophetic statements and an active assessment of them while the latter generation is a move towards simply referencing a book or citing a text.
Fourth century preservation and split between the jurists and traditionists
Reliant on books, ḥadīth now came to be canonised. Two things took place in this generation: one observes that the science of principle of ḥadīth, usūl ḥadīth, becomes a form of complaint against students who, not having truly struggled in the search for ḥadīth collections, are not the same calibre as the previous generation of scholars. Al-Khaṭtābī, in his introduction to Ma’ālim al-Sunan Ṣharh Sunnan Abī Dawūd, laments:
فقد عاد الدين غريباً كما بدأ وعاد هذا الشأن دارسة أعلامه خاوية أطلاله وأصبحت رباعه مهجورة ومسالك طرقه مجهولة
In our days, religion has once again become something alien, as it was when it began; its signposts have been obliterated, the remains of its encampments are empty, its quarters deserted, and the paths to its roads unknown. 
He also mentions another crucial split: between the jurists, or fuqahā and the traditionists, or muhadithīn. It is worth dwelling on this point a little bit more:
ورأيت أهل العلم في زماننا قد حصلوا حزبين وانقسموا إلى فرقتين أصحاب حديث وأثر، وأهل فقه ونظر
I see the scholars in our time divided into two parties and split into two groups:  The people of the ḥadīth and the prophetic heritage, and  The people of jurisprudence (fiqh) and reflective reasoning.
He critiques both camps beginning with the scholars of ḥadīth:
فأما هذه الطبقة الذين هم أهل الأثر والحديث فإن الأكثرين منهم إنمّا وكدهم الروايات وجمع الطرق وطلب الغريب والشاذ من الحديث الذي أكثره موضوع أو مقلوب لا يراعون المتون ولا يتفهمون المعاني ولا يستنبطون سيرها ولا يستخرجون ركازها وفقهها وربما عابوا الفقهاء وتناولوهم بالطعن وادعوا عليهم مخالفة السنن ولا يعلمون أنهم عن مبلغ ما أوتوه من العلم قاصرون وبسوء القول فيهم آثمون.
Their interests only lie in:
…the transmissions, the collecting of paths [of transmission], and searching for the uncommon and irregular [accounts and expressions] of the Prophetic tradition, most of which are either fabricated (mawdū’) or distorted (maqlūb )…
They fail to study the ḥadīth tradition seriously or examine the content and meaning critically and end up defaming the jurists by accusing them of diverging from the normative traditions of the Prophet, may Allah peace and blessings be upon him:
…[In so doing,] they (i.e., the scholars ḥadīth) do not realise that they cannot reach the level of knowledge which they (i.e., the jurists) have attained, and that they (i.e., the scholars of ḥadīth) are sinning by defaming them.
His concern lies with trying to bring both camps together and recognising that they both depend on each other, since ḥadīth is the foundation and root of religion while fiqh is the building upon which it rests:
ورأيت أهل العلم في زماننا قد حصلوا حزبين وانقسموا إلى فرقتين أصحاب حديث وأثر، وأهل فقه ونظر، وكل واحدة منهما لا تتميز عن أختها في الحاجة ولا تستغني عنها في درك ما تنحوه من البغية والإرادة، لأن الحديث بمنزلة الأساس الذي هو الأصل، والفقه بمنزلة البناء الذي هو له كالفرع، وكل بناء لم يوضع على قاعدة وأساس فهو منهار، وكل أساس خلا عن بناء وعمارة فهو قفر وخراب
Indeed, any structure that does not have a firm foundation is prone to collapse while any foundation without its structure is as a good as a ruin. As for the jurists, they fail to distinguish between the various categories of ḥadīth and are guilty of using weak ones as evidence when debating with their opponents so long as it in one way or another agrees with their school:
وأما الطبقة الأخرى وهم أهل الفقه والنظر فإن أكثرهم لا يعرجون من الحديث إلاّ على أقله ولا يكادون يميزون صحيحه من سقيمه، ولا يعرفون جيده من رديئه ولا يعبؤون بما بلغهم منه أن يحتجوا به على خصومهم إذا وافق مذاهبهم التي ينتحلونها ووافق آراءهم التي يعتقدونها وقد اصطلحوا على مواضعة بينهم في قبول الخبر الضعيف والحديث المنقطع إذا كان ذلك قد اشتهر عندهم.
Al-Khaṭtābī then moves on to back up his claims by citing examples from the four schools:
وهؤلاء وفقنا الله وإياهم لو حكي لهم عن واحد من رؤساء مذاهبهم وزعماء نحلهم قول يقوله باجتهاد من قبل نفسه طلبوا فيه الثقة واستبرؤوا له العهدة. فتجد أصحاب مالك لا يعتمدون من مذهبه إلاّ ما كان من رواية ابن القاسم والأشهب وضربائهم من تلاد أصحابه فإذا وجدت رواية عبد الله بن عبد الحكم وأضرابه لم تكن عندهم طائلاً. وترى أصحاب أبي حنيفة لا يقبلون من الرواية عنه إلاّ ما حكاه أبو يوسف ومحمد بن الحسن والعلية من أصحابه والأجلة من تلامذته فإن جاءهم عن الحسن بن زياد اللؤلؤي وذويه رواية قول بخلافه لم يقبلوه ولم يعتمدوه.وكذلك تجد أصحاب الشافعي إنما يعولون في مذهبه على رواية المزني والربيع بن سليمان المرادي فإذا جاءت رواية حرملة والجيزي وأمثالهما لم يلتفتوا إليها ولم يعتدوا بها في أقاويله. وعلى هذا عادة كل فرقة من العلماء في أحكام مذاهب أئمتهم وأستاذيهم.
Al-Khaṭtābī forcefully argues that if they maintain such standards of reliability in positive law, or furū’, when it comes to ascribing statement to their ‘masters’ of their school, then how is it possible, or even for them to become indifferent, when it comes to the communication and the transmission of the Prophet, may the peace and blessings of God be upon him:
فإذا كان هذا دأبهم وكانوا لا يقنعون في أمر هذه الفروع وروايتها عن هؤلاء الشيوخ إلاّ بالوثيقة والثبت فكيف يجوز لهم أن يتساهلوا في الأمر الأهم والخطب الأعظم وأن يتواكلوا الرواية والنقل عن إمام الأئمة ورسول رب العزة، الواجب حكمه اللازمة طاعته، الذي يجب علينا التسليم لحكمه والانقياد لأمره من حيث لا نجد في أنفسنا حرجاً مما قضاه ولا في صدورنا غلاً من شيء مما أبرمه وأمضاه.
However, both group of scholars seem to get off lightly when we look at his criticism of another group, the speculative theologians, or the mutakallimūn:
ولكن أقواماً عساهم استوعروا طريق الحق واستطالوا المدة في درك الحظ وأحبوا عجالة النيل فاختصروا طريق العلم واقتصروا على نتف وحروف منتزعة عن معاني أصول الفقه سموها عللاً وجعلوها شعاراً لأنفسهم في الترسم برسم العلم واتخذوها جُنّة عند لقاء خصومهم ونصبوها دريئة!!! للخوض والجدال يتناظرون بها ويتلاطمون عليها، وعند التصادر عنها قد حكم للغالب بالحذق والتبريز فهو الفقيه المذكور في عصره والرئيس المعظم في بلده ومصره هذا وقد دسّ لهم الشيطان حيلة لطيفة وبلغ منهم مكيدة بليغة. فقال لهم هذا الذي في أيديكم علم قصير وبضاعة مزجاة لا تفي بمبلغ الحاجة والكفاية فاستعينوا عليه بالكلام وصلوه بمقطعات منه واستظهروا بأصول المتكلمين يتسع لكم مذهب الخوض ومجال النظر، فصدق عليهم ظنه وأطاعه كثير منهم واتبعوه إلاّ فريقاً من المؤمنين. فيا للرجال والعقول أنّى يذهب بهم وأنّى يختدعهم الشيطان عن حظهم وموضع رشدهم والله المستعان.
However, there are people who may find the path to the truth rough, and the time to gain fortune long if they love speedy gain. Thus they shorten the path to knowledge and content themselves with bits and pieces detached from the sense of the foundations of jurisprudence, calling them “causes” (‘ilal ). They take them as banners for themselves in pretending to be knowledgeable and use them as a shield in their meetings with their opponents, and as a target for them [in order] to start discussing and arguing [with them] in debates and clashes over them. At the end of the discussion, the “winner” will have been accredited with “astuteness” and “superiority.” He is said to be the “distinguished jurisconsult of his time”, the “glorified leader of his country and region.” However, with this the Devil has already introduced a “fine trick” to them and drawn them into a grave plot. He (i.e., this so called “distinguished jurist”) then says to them: “That, which is in your hands, is [only] insignificant knowledge and an inferior commodity; it does not reach the level of what is “necessary” and “satisfactory” [in this regard]. Therefore, you should seek the assistance of kalām, add a few items thereof, and seek the support of the foundations of the knowledge of the speculative theologians!
With this, the method of examination) and the field of rationalist reflection will increase!” He (i.e., the “mischievous jurist”) was right in his assumption about them. Many of them obeyed and followed him, with the exception of one group of believers. Oh, what men and what intellects! Where the Devil leads them and how he tricks them out of their luck and their intelligence! It is only Allah who grants help!
And only Allah knows best.
 See Muqaddimah ibn al-Ṣalāḥ fī ‘Ulūm al-Ḥadīth.
 It is worth noting that many of entries (ma‘ājim) are not companions and there remains debate as to whether they are or are not. Out of the many companions, the ‘narrator’ companions do not exceed two thousand.
 Examples include the Muwaṭṭa of Imām Mālik, Muṣannaf ibn Jurayj, Kitāb al-Athār while the latter ones, such as the Muṣannaf of Abd al-Razzāq and Muṣannaf of Ibn Abī Ṣhaybah, were more comprehensive.
 For a more detailed discussion see, Gunther, S. (2008). In our days, religion has once again become something alien: Al-Khattabi’s Critique of the State of Religious Learning in Tenth century Islam. American Journal of Muslim Social Scientists, (25-3), 1-30. I have relied on much of the translation from this paper with a few minor edits where necessary.