To which era did Ibn Ḥajar belong?
One of the challenges any student of knowledge faces is trying to remember when scholars were around. So when it comes to somone like Ibn Ḥajar Al-Asqalāni, one can, by recalling his work, Al-Durar Al-Kāmina Fī A’yāni Al-Miati Al-Thāmina, (the hidden gem of notables in to 8th century) quickly come to know when he was around. The same can apply to his student, Ṣhamṣ al-Dīn Muḥammad ibn ‘Abd al-Raḥmān al-Sakhāwī, from his text, Al-Ḍaw’ al-lāmi’ li ahli al-Qarni al-Tāsi (the splendid light of notables in the 9th century). Finally, one can for example, by recalling An-nūr Al-Sāfir an Akhbār Al-Qurn Al-Āa’shir (the manifest light regarding reports of the 10th century) know when its author, Abdul Qādir bin Abdullāh al-Aydarūs was around. This was just a handy tip. Anyway, lets move on to what we really want to discuss.
When did ḥadīth criticism begin?
Before attempting to provide an answer to this question, one ought to ask more generally: does the law come first or society? Or do they both come together? In other words, does society come into operation followed by the law which them aims to regulate behaviour, or, are laws expounded with society then expected to conform to it? There are three theories propounded in response to this question.
The first states that the law or regulation of society comes later; much in the same way it is argued that the corpus of Ḥanafi law emerged out of society; or the way rules for Arabic grammar emerged after the spread of Islam to non-Muslim lands. The second theory states opposite; that the law comes first. For example, the scholars of ḥadīth cite the verse of the Qur’ān: “O you who believe! If an iniquitous person comes to you with tidings, then be discerning…” The argument goes that the only way to know if someone is iniquitous or honest is to verify their character. Furthermore, the entire Qur’ān is actively engaged in grounding its readers in observing the character of people. Therefore, the character of a person is paramount. Here we see the law providing guidance as to how society should behave; law structures society.
The final theory is that they [both law and society] emerged together. This can be gleaned from the wife of the Prophet, may the peace and blessings of God be upon him, Ḳhadijāh, may God be pleased with her, who, when informed of the first revelation, sent for her cousin Waraqāh bin Nawfal. This incident shows that she had already made a ‘judgement’ based on a number of considerations (e.g. believing in his integrity and honesty). The same can be understood from Abu Bakr, may God be pleased with, and his belief in the night journey of the Prophet, may the peace and blessings of God be upon him. For them, the truth of the Prophets message was a foregone conclusion. In both of these examples, we can see that society and law emerged together.
If we develop the third theory, we see that the companions were actively engaged in ḥadīth criticism. For example, Ibn Mas’ūd, may God be pleased with him, was known to be stringent in narrating ḥadīth. This theory, then, helps to explain why despite that fact that there were so many companions, the collection of actual ḥadīth is much smaller; there was a ‘filter’ in process. More accurately, they were engaging in the science of verification (tathabut). It was Umar, may God be pleased with him, who institutionalised this process in order to gain certainty over ḥadīth. The narration of Umar verifying the ḥadīth of knocking on a door three times is a well-known example of this. To summarise, transmission and criticism went hand in hand right from the early days of Islam. This, however, was counterbalanced by ‘incentives’ to narrate ḥadīth. So for example, we find the Prophet, may the peace and blessings of God be upon him, reported as saying, ‘Convey from me, even if it is one verse.’
Moving on, when we look at narrator (rāwī) we look for two things: uprightness or integrity (adālah); and, memory (ḍhabt). As for the former, it is the belief of the Sunnīs that none of the Companions deliberately lied about the Prophet, may the peace and blessings of God be upon him. However, this does not discount that they may have been wanting when it comes to memory. Some examples will serve to illustrate this point.
As for the night journey (al-Mi’rāj) of the Prophet, may the peace and blessings of God be upon him, Ibn Abbās was of the opinion that the Prophet, saw God. Ā’iṣhah, may God be pleased with her, however, for example, was of a different opinion. Ibn Umar from his father, may God be pleased with them both, mentions that the Prophet may the peace and blessings of God be upon him said, ‘The deceased is tormented in his grave because of the lamentations (wailing) over him.’ However, again we find, Ā’iṣhah, may God be pleased with her, performed ijtihād on her part, because she thought that they contradicted the words of God, ‘and no bearer of burdens shall bear the burden of another.’ For our purposes, she says, ‘…by God you are not narrating this ḥadīth from two liars [Ibn Umar and Umar] who have disbelieved, but sometimes you mishear.’
Abū Hūrairah, may God be pleased with him, was of the opinion that one needed to perform ablution (wuḍhū) from that which has been touched by the fire. Ibn Abbās, may God be pleased with him, used analogical reasoning (qiyās) and asked him what would one do with water that had been heated since this would lead to an absurdity. The point is, despite there being any number of explanations for the the statements made by the Companions, they were always actively engaged in critiquing one another, though their integrity was never in question.
And only God knows best.
 Qur’ān 49:7
 They said to Abū Bakr, ‘Look at what your companion is saying. He says he went to Jerusalem and came back in one night.’ Abū Bakr, told them, ‘If he said that, then he is truthful. I believe him concerning the news of the heavens–that an angel descends to him from the heavens. How could I not believe he went to Jerusalem and came back in a short period of time–when these are on earth?’ At that, the Companion, Abū Bakr, was called “al-Ṣiddīq”—because of how strongly he believed all what the Prophet, may the peace and blessings of God be upon him, said.
 In fact, Umar was doing this during the Prophet, Prophet, may the Peace and Blessings of God be Upon Him, time when for example, he went to him to verify if he had indeed divorced his wives (See Bukhārī). A second example is that of Ḍhimām bin Ṭha’labah: It was narrated from ṢharĪk bin Abdullāh bin Abū Namir that he heard Anas bin Mālik say: ‘While we were sitting in the mosque, a man entered riding a camel; he made it kneel in the mosque, then he hobbled it and said to them: ‘Which of you is Muḥammad?’ The Prophet, may the peace and blessings of God be upon him, was reclining among them, so they said: ‘This fair- skinned man who is reclining.’ The man said to him: ‘O son of Abdul Muṭalib!’ The Prophet, may the peace and blessings of God be upon him, said: ‘I am listening to you.’ The man said: O Muḥammad! I am asking you and will be stern in asking, so do not bear any ill-feelings towards me.’ He said: ‘Ask whatever you think.’ The man said: ‘I adjure you by your Lord and the Lord of those who came before you, has God sent you to all of mankind?’ The Prophet, may the peace and blessings of God be upon him, said: ‘By God, yes.; He said: ‘I adjure you by God, has God commanded you to pray the five prayers each day and night?’ The Prophet, may the peace and blessings of God be upon him, said: ‘By God, yes.’ He said: ‘I adjure you by God, has God commanded you to fast this month of each year?’ The Prophet, may the peace and blessings of God be upon him, said : ‘By God, yes.’ He said: ‘I adjure you by God, has God commanded you to take this charity from our rich and distribute it among our poor?’ The M Prophet, may the peace and blessings of God be upon him, said: ‘By God, yes.’ The man said: ‘I believe in what you have brought, and I am the envoy of my people who are behind me. I am Ḍhimām bin Ṭha’labah, the brother of Banū Sā’d bin Bakr (see Muslim).
Imām Al-Ḍhahabi mentions in Tazkiratul Hufāz, ‘he [Umar] was the first one to establish [a precedent of] verification in narration.’ Elsewhere, it was said by Muāwiyah, may God be pleased with him, on the pulpit, ‘O people, be careful over the narrations of the Prophet, may the peace and blessings of God be upon him, except for the narrations mentioned during the time of Umar, may God be pleased with him, for he used to interrogate people with the fear of God…’ (See Tārīkh Madīnah Dimiṣhq)
 Once Abū Mūsā went to Umar’s house and knocked three times. When he did not reply, Abū Mūsā turned to leave. Umar came out and asked, ‘Where are you going?’ Abū Mūsā replied that the Prophet, may the peace and blessings of God be upon him, had told him to leave after three knocks. Umar told him to find another companion to witness what he was saying or Umar would beat him. Abū Mūsā went to the masjid and found a group of Ṣaḥābāh. He told them what happened, and they said, SubḥānAllāh, Umar doesn’t know this ḥadīth! They sent the youngest amongst them – Abū Sa’īd al-Khūdrī – to tell Umar. (see Bukhāri)
 It was narrated that Ā’iṣhah, may God be pleased with her said, ‘Whoever told you that Muḥammad, may the peace and blessings of God be upon him, saw his Lord was lying. He said that no vision can grasp him’ (Bukhāri). However, It was narrated that Ibn Abbās said, the ‘(Prophet’s) heart belied not what he saw, and indeed he saw Him at a second descent.’ [Qur’ān 53:11-12] (This means that) he saw Him twice with his heart’ (Muslim). See Ibn Kathīr for all the different opinions on this.
 Bukhāri and Muslim.
 Qur’ān 6:164
 It was narrated from Abū Hūrairah that the Prophet, may the peace and blessings of God be upon him, said, ‘perform ablution after (eating) that which has been changed by fire.’ Ibn Abbās said: ‘should I do ablution after (touching) hot water?” Abū Hūrairah said: ‘O son of my brother, when I narrate a ḥadīth of the Messenger of God, may the peace and blessings of God be upon him, to you, then do not try to make examples for it.’ (Ibn Mājah)