Shia Islam is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib (656–661 CE) as his successor as Imam, most notably at the event of Ghadir Khumm, but that after Muhammad's death, Ali was prevented from succeeding as leader of the Muslims as a result of the choice made by some of Muhammad's other companions at Saqifah. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abu Bakr, who was appointed caliph by a group of Muhammad's other companions at Saqifah, to be the first Rashidun ('rightful') caliph after Muhammad (632–634 CE).