The point of this entire speech is that one of the reasons for the differing of the Four Imams, Abū Ḥanīfah, Mālik, al-Shāfi’ī, and Ahmad, is, as I have told you, that the Sunnah was spread about due to the spreading about of those who bore it, the Companions and the tābi‘ūn.
Then came the role of the Four Imams mentioned here.
The first one of them, as you know, is Abū Ḥanīfah (may Allah have mercy on him). He died in the year 150 AH, so he is considered one of the ṣighār (junior) tābi‘ūn. Abū Ḥanīfah (may Allah have mercy on him) specialized in fiqh (Islamic jurisprudence) and never left al-Kūfah (Kufa in Iraq) except to perform ḥajj or ‘umrah.
Thus, he had a small inventory of ḥadīth.
Following him was Imam Mālik. He would later reside in the land of hijrah, Madinah. As is also the case today, Madinah, being an attraction for pilgrims of the ḥajj and ‘umrah, has been home to many scholars. As a result, Imam Mālik likewise did not travel to compile ḥadīth.
He did however, retain more ḥadīth than Abū Ḥanīfah.
Then came the third imam, Imam al-Shāfi’ī (may Allah have mercy on him). He resided in Baghdad, which was the capital of the Abbasid caliphate, as you know. It was also a resort for various scholars and was blessed with as much knowledge and Sunnah as Allah willed.
He left Baghdad for Egypt, and as a result, gained new material for knowledge − the knowledge of ḥadīth, in particular. So, he surpassed the first two imams, Imam Abū Ḥanīfah and Imam Mālik, because he spent a significant part of his life in Baghdad in addition to performing ḥajj and ‘umrah and spending the later part of his life in Egypt. So he was able to gather ḥadīths that both of the earlier imams were not.
The fourth imam, Aḥmad ibn Ḥanbal, came and surpassed not only the first three imams but the masses of ḥadīth scholars due to his comprehensive knowledge of the Sunnah. He retained more of the Sunnah and had more knowledge in ḥadīth than most ḥadīth scholars.
Owing to this, scholars like Abū Ḥanīfah would receive questions and refer back to whatever of the Qur’ān and ḥadīth of the Messenger of Allah (ṣallá’llāhu ‘alayhi wa sallam) were available to them and would give a fatwá (an Islamic verdict) according to that. If nothing was available, one would resort to ijtihād (independent reasoning based on the rulings of the Quran and Sunnah).
Ijtihād is necessary, as the Prophet (ṣallá’llāhu ‘alayhi wa sallam) said: “When a judge gives a decision, having tried his best to decide correctly and is right, there are two rewards for him; and if he gave a judgment after having tried his best (to arrive at a correct decision) but erred, there is one reward for him.” [Translation: Siddiqui] But, as you understand from this ḥadīth, ijtihād is susceptible to being right or wrong, so one could ask other imams the same question and receive an answer based on the ḥadīth available to that imam, ḥadīth which may not have been available to another imam who rendered his fatwá based on ijtihād.
Knowing this historical fact in relation to the compilation of ḥadīth should remove any confusion in our minds about the differing of the imams. Shaykh Muhammad Nassir Adeen Al-Albaani in Al-Albaani’s Encyclopedia of Manhaj