The imams based their legitimacy on descent from the Islamic prophet Muhammad, mostly via al-Qasim ar-Rassi (d. 860). After him, the medieval imams are sometimes known as the Rassids. The first of the ruling line, his grandson al-Hadi ila'l-Haqq Yahya, was born in Medina. His fame as an intellectual as well as a leader of note, led to his invitation to Yemen. He was summoned to govern the highland tribes in 893 and again in 896–97. Al-Hadi introduced a multitude of policies and practices that evolved into the particular Yemenite Zaidiyyah brand. The efforts of al-Hadi eventually became the basic guidelines for the religious as well as political characteristics of Yemeni Zaydism. Al-Hadi, however, was not able to consolidate his rule in all of Yemen. He could not even create an enduring state in the highlands, due to the strong localism persisting in the region. There were revolts as well as segments of the population that did not accept his and his successors' pretensions to religio-political rule.Cornelis van Arendonk, Les débuts de l'imamat zaidite au Yemen. Leiden 1960
