Nuʿman ibn Thabit ibn Zuta ibn Marzuban, better known as Abu Hanifah (rahimahullaah) (699 — 765 CE / 80 — 148 AH) was was born in Kufa, Iraq. He was the first in order of the Four Great Imaams whose companions have narrated from him various sayings and diverse warnings, all of them leading to one thing: the obligation to accept the Qur'an and Hadeeth only, and to give up following the opinions of the imaams which contradict it:

1. "When a hadeeth is found to be saheeh, then that is my madhhab."

Ibn 'Aabideen in al-Haashiyah (1/63), and in his essay Rasm al-Mufti (1/4 from the Compilation of the Essays of Ibn 'Aabideen), Shaikh Saalih al-Fulaani in Eeqaaz al-Himam (p. 62) & others. Ibn 'Aabideen quoted from Sharh al-Hidaayah by Ibn al-Shahnah al-Kabeer, the teacher of Ibn al-Humaam, as follows:
"When a hadeeth contrary to the Madhhab is found to be saheeh, one should act on the hadeeth, and make that his madhhab. Acting on the hadeeth will not invalidate the follower's being a Hanafi, for it is authentically reported that Abu Haneefah said, 'When a hadeeth is found to be saheeh, then that is my madhhab', and this has been related by Imaam Ibn 'Abdul Barr from Abu Haneefah and from other imaams."

This is part of the completeness of the knowledge and piety of the Imaams, for they indicated by saying this that they were not versed in the whole of the Sunnah, and Imaam Shaafi'i (rahimahullaah) has elucidated this thoroughly. It would happen that they would contradict a sunnah because they were unaware of it, so they commanded us to stick to the Sunnah and regard it as part of their Madhhab. May Allaah shower His mercy on them all.

2. "It is not halaal (permitted) for anyone to accept our views if they do not know from where we got them."

Ibn 'Abdul Barr in Al-Intiqaa' fi Fadaa'il ath-Thalaathah al- A'immah al-Fuqahaa' (p. 145), Ibn al-Qayyim in I'laam al- Mooqi'een (2/309), Ibn 'Aabideen in his Footnotes on Al-Bahr ar-Raa'iq (6/293) and in Rasm al-Mufti (pp. 29,32) & Sha'raani in Al-Meezaan (1/55) with the second narration. The last narration was collected by 'Abbaas ad-Dawri in At- Taareekh by Ibn Ma'een (6/77/1) with a saheeh sanad on the authority of Zafar, the student of Imaam Abu Haneefah (rahimahullaah). Similar narrations exist on the authority of Abu Haneefah's companions Zafar, Abu Yoosuf and 'Aafiyah ibn Yazeed; cf. Eeqaaz (p. 52). Ibn al-Qayyim firmly certified its authenticity on the authority of Abu Yoosuf in I'laam al-Mooqi'een (2/344). The addition to the second narration is referenced by the editor of Eeqaaz (p. 65) to Ibn 'Abdul Barr, Ibn al-Qayyim and others.

If this is what they say of someone who does not know their evidence, what would be their response to one who knows that the evidence contradicts their saying, but still gives verdicts opposed to the evidence?! Therefore, reflect on this saying, for it alone is enough to smash blind following of opinion.

In one narration, "It is haraam (prohibited) for someone who does not know my evidence to give fatwaa (verdicts) on the basis of my words."

Another narration adds, "... for we are mortals: we say one thing one day, and take it back the next day."

In another narration, "Woe to you, O Ya'qub (i.e. Imaam Abu Haneefah's illustrious student, Abu Yoosuf (rahimahullaah))! Do not write down everything you hear from me, for it happens that I hold one opinion today and reject it tomorrow, or hold one opinion tomorrow and reject it the day after tomorrow."

This was because the Imaam would often base his view on Qiyaas (Analogy), after which a more potent analogy would occur to him, or a hadeeth of the Prophet (صلى الله عليه وسلم) would reach him, so he would accept that and ignore his previous view. Sha'raani's words in Al-Meezaan (1/62) are summarised as:
"Our belief, as well as that of every researcher into Imaam Abu Haneefah is that, had he lived until the recording of the Sharee'ah, and the journeys of the Preservers of Hadeeth to the various cities and frontiers in order to collect and acquire it, he would have accepted it and ignored all the analogies he had employed. The amount of qiyaas in his Madhhab would have been just as little as that in other Madhhabs, but since the evidences of the Sharee'ah had been scattered with the Successors and their successors, and had not been collected in his lifetime, it was necessary that there be a lot of qiyaas in his Madhhab compared to that of other imaams. The later scholars then made their journeys to find and collect ahaadeeth from the various cities and towns and wrote them down; hence, some ahaadeeth of the Sharee'ah explained others. This is the reason behind the large amount of qiyaas in his Madhhab, whereas there was little of it in other Madhhabs."

Abul-Hasanaat Al-Lucknowi quoted his words in full in An- Naafi' al-Kabeer (p. 135), endorsing and expanding on it in his footnotes, so whoever wishes to consult it should do so there.

Since this is the justification for why Abu Haneefah (rahimahullaah) has sometimes unintentionally contradicted the authentic ahaadeeth - and it is a perfectly acceptable reason, for Allaah does not burden a soul with more than it can bear - it is not permissible to insult him for it, as some ignorant people have done. In fact, it is obligatory to respect him, for he is one of the imaams of the Muslims through whom this Deen has been preserved and handed down to us, in all its branches; also, for he is rewarded under any circumstance: whether he is correct or wrong. Nor is it permissible for his devotees to continue sticking to those of his statements which contradict the authentic ahaadeeth, for those statements are effectively not part of his Madhhab, as the above sayings show. Hence, these are two extremes, and the truth lies in between.

"Our Lord! Forgive us, and our brethren who came before us into the Faith; and leave not, in our hearts, any rancour against those who have believed. Our Lord! You are indeed Full of Kindness, Most Merciful."

[Al Qur'an 59:10]

3. "When I say something contradicting the Book of Allaah the Exalted or what is narrated from the Messenger (صلى الله عليه وسلم), then ignore my saying."

Al-Fulaani in Eeqaaz al-Himam (p. 50), tracing it to Imaam Muhammad and then saying, "This does not apply to the mujtahid, for he is not bound to their views anyway, but it applies to the muqallid."

Sha'raani expanded on that in Al-Meezaan (1/26):
"If it is said: 'What should I do with the ahaadeeth which my Imaam did not use, and which were found to be authentic after his death?' The answer which is fitting for you is: 'That you act on them, for had your Imaam come across them and found them to be authentic, he would have instructed you to act on them, because all the Imaams were captives in the hand of the Sharee'ah.' He who does so will have gathered all the good with both his hands, but he who says, 'I will not act according to a hadeeth unless my Imaam did so', he will miss a great amount of benefit, as is the case with many followers of the Imaams of the Madhhabs. It would be better for them to act on every hadeeth found to be authentic after the Imaam's time, hence implementing the will of the Imaams; for it is our firm belief about the Imaams that had they lived longer and come to know of those ahaadeeth which were found authentic after their time, they would have definitely accepted and acted according to them, ignoring any analogies they may have previously made, and any views they may have previously held."