Where and when did the notion of "You must follow one madhhab exclusively" originate?

The notion of following a single madhhab (school of thought) exclusively in Islamic jurisprudence developed over time, particularly during the later stages of Islamic history. Initially, the early Muslim community was more flexible, with various companions of the Prophet Muhammad ﷺ expressing differing interpretations of Islamic law. However, as the Islamic world expanded and diverse opinions emerged, scholars began to categorize these interpretations into distinct schools of thought.

One significant factor in promoting adherence to a single madhhab was the emergence of formalized schools of jurisprudence in the 2nd and 3rd centuries of the Islamic calendar (approximately the 8th to 9th centuries CE), coinciding with the establishment of the four main Sunni madhhabs: Hanafi, Maliki, Shafi'i, and Hanbali. The scholars of these schools emphasized the importance of following their particular methodologies for deriving legal rulings, which included the Qur'an, hadith, consensus (ijma), and analogy (qiyas).

While the Qur'an itself does not explicitly command followers to adhere to one madhhab, it emphasizes the importance of unity and following guidance. For instance, Surah Ash-Shura states: And be thou upright as thou art commanded, and follow not their lusts, but say: I believe in whatever scripture Allah hath sent down… 42:15. This verse underscores the need for adherence to divine guidance and just conduct.

Furthermore, the historical context, including the fragmentation of the Muslim community into various sects, led scholars to advocate for a more unified approach to jurisprudence. This was reflected in the hadith where the Prophet ﷺ mentioned that his Ummah would split into seventy-three sects, with only one being saved (as noted in the commentary of Imam Ahmad). This encouraged scholars to seek a singular path of practice within a madhhab to maintain cohesion among Muslims.

In summary, while the call to follow one madhhab exclusively is not directly stated in the Qur'an, historical developments and the need for legal uniformity among the Muslim community led to the emphasis on adherence to specific madhhabs in Islamic jurisprudence.