al-Hadi ila al-Haqq Yahya

Yahya ibn al-Husayn, known by the regnal title al-Hadi ila al-Haqq ("the Guide to the Truth"), was the founder of the Zaydi Shia imamate in Yemen and one of the most influential jurists of the Zaydi branch of Shia Islam. Zaydis are the Shia community who hold that the imamate belongs to a learned, activist descendant of Ali and Fatima who openly claims it — a doctrine that shaped al-Hadi's own career. He was a grandson of al-Qasim al-Rassi, a leading early Zaydi authority, and was born around 245/859 in the region of Medina, in the Hijaz; Zaydi tradition names Medina itself, though modern scholars place his birth at a village near the wadi al-Rass where his family had settled.

After an earlier period that, according to the sources, included travel to the Caspian province of Tabaristan, al-Hadi was invited by northern Yemeni tribes seeking relief from Abbasid governors and local strongmen. He entered Sa'da in 897 and proclaimed his daʿwa (call to allegiance), making the town the seat of an imamate that, in various forms, persisted in Yemen until 1962. He repeatedly captured the larger city of Sana'a but was never able to hold it securely. He died in Sa'da in 298/911. His legal and theological writings — above all the Kitab al-Ahkam — became the foundation of the Hadawi madhhab (school of law), which has dominated Zaydi practice in Yemen ever since. Reports describe his theology as leaning toward Mu'tazili rationalism.