Sunnis and Shias disagree over the rightful successor to Prophet Muhammad. Sunnis believe that the first three caliphs who succeeded the Prophet - Abu Bakr, Umar, and Uthman - were rightfully appointed as leaders of the Islamic community, while Shias believe that Ali, the Prophet's cousin and son-in-law, should have been the immediate successor. This disagreement led to the split between Sunnis and Shias in the early days of Islam.
