Ali al-Rida

Ali al-Rida was the eighth Imam of Twelver Shia Islam, a descendant of the Prophet Muhammad born in Medina, who succeeded his father Musa al-Kazim in 799. Departing from the anti-Shia policy of his predecessors, the Abbasid caliph al-Ma'mun invited him to Merv in Khorasan and in 817 named him heir apparent, changing the official colour from Abbasid black to green. The appointment provoked an Abbasid revolt in Baghdad, and as the court travelled back to Iraq al-Rida died at Tus in 818, probably poisoned; many historians hold al-Ma'mun responsible. The city of Mashhad grew up around his grave, the holiest site in Iran, visited by millions of Shia pilgrims every year.