Kitab al-Ahkam (The Book of Rulings)

Al-Hadi ila'l-Haqq Yahya b. al-Husayn was born in 859, more than a century after Zayd ibn Ali's own rising and death had already given the Zaydi movement its name. In 897 he proclaimed his own religious and political mission at Sa'da, in the mountains of northern Yemen, and there founded the first lasting Zaydi imamate, a state that traced its legitimacy through the same principle Zaydis had held from the start, that the rightful leader of the community must be a qualified descendant of Ali who is willing to rise and claim the position openly, not merely wait for it in seclusion.

Governing that new imamate meant ruling on real cases, and al-Hadi set his own legal reasoning down in a book he called Kitab al-Ahkam, the Book of Rulings. He never finished it. When he died in 911, after fourteen years as imam, the work remained incomplete, and it fell to his follower Muhammad ibn Sulayman al-Kufi to compile a second volume, Kitab al-Muntakhab, gathering further rulings attributed to his teacher to round out what the unfinished Ahkam had left open. Between the two books, an imamate founded in Sa'da less than two decades earlier now had a body of law written down in its own founder's name, unfinished or not, and that law became the seed of a distinct legal tradition that would carry al-Hadi's own title, al-Hadi ila'l-Haqq, forward as its name: the Hadawiyya.

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.