Al-Hadi ila'l-Haqq Yahya, first Zaydi Imam of Yemen (born 859)

911 (CMXI) was a common year starting on Tuesday of the Julian calendar.

Abūʾl-Ḥusayn Yaḥyā ibn al-Ḥusayn ibn al-Qāsim ibn Ibrāhīm Ṭabāṭabā al-Ḥasanī, better known by his honorific title of al-Hādī ilāʾl-Ḥaqq, was a religious and political leader in the Arabian Peninsula. He was the first Zaydi imam who ruled portions of Yemen from 897 to 911. He is also the ancestor of the Rassid Dynasty which ruled Yemen intermittently until the North Yemen Civil War in 1962.

Zaydism, also referred to as Fiver Shi'ism, is a branch of Shia Islam that emerged in the eighth century following Zayd ibn Ali's unsuccessful rebellion against the Umayyad Caliphate. Zaydism is one of the three main branches of Shi'ism, with the other two being Twelverism and Ismailism.

The Imams of Yemen, later also titled the Kings of Yemen, were religiously consecrated leaders (imams) belonging to the Zaidi branch of Shia Islam. They established a blend of religious and temporal-political rule in parts of Yemen from 897. Their imamate endured under varying circumstances until the end of the North Yemen civil war in 1970, following the republican revolution in 1962. Zaidi theology differs from Isma'ilism and Twelver Shi'ism by stressing the presence of an active and visible imam as leader. The imam was expected to be knowledgeable in religious scholarship, and to prove himself a worthy headman of the community, even in battle if this was necessary. A claimant of the imamate would proclaim a "call" (dawah), and there were not infrequently more than one claimant.