Two Books, Two Eras, One Word for Both

Zaidiyyah traces its name to Zayd ibn Ali, whose failed rising against Umayyad rule in 740 gave the movement its founding figure a century and a half before al-Hadi ila'l-Haqq Yahya was even born. Zayd's own legal corpus survives as Musnad al-Imam Zayd, compiled by his disciple Abu Khalid al-Wasiti and published in a modern critical edition under the Latin title Corpus Iuris di Zaid b. Ali, also known by the Arabic name Majmu al-Fiqh. It is an early document of Islamic law in its own right, reflecting the wider legal thinking of Kufa in Zayd's own time as much as any single doctrine of his.

Al-Hadi's Kitab al-Ahkam is a different book from a different, later moment. Where Zayd's corpus is the record of an early Kufan legal milieu his rising happened to be associated with, al-Hadi wrote as a ruling imam actually governing a state he had just founded, setting down rulings to meet the practical needs of Sa'da's new imamate as he encountered them, work his own death left unfinished. The Yemeni Zaydi tradition that grew from his rulings, the Hadawiyya, remains distinct enough from the older Kufan corpus that scholars treat the two as separate stages in Zaydi legal history rather than one continuous text, even though both carry weight within the same wider Zaydi tradition and both, in their own centuries, did the same job: giving a movement built around a founding figure's rising a body of law to govern by.