Shia Islam is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated Ali as his successor (khalifa) and the Imam after him, most notably at the event of Ghadir Khumm, but was prevented from succeeding Muhammad as the leader of the Muslims as a result of the choice made by some of Muhammad's other companions (sahaba) 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 rightful (rashidun) caliph after Muhammad. Adherents of Shia Islam are called Shia Muslims.