One main point of disagreement between Sunni and Shia Muslims is their differing beliefs about the rightful successor to the Prophet Muhammad. Sunnis believe that Abu Bakr, one of Muhammad's companions, was the rightful successor, while Shias believe that Ali, Muhammad's cousin and son-in-law, was the rightful successor. This disagreement has led to centuries of sectarian tension and conflict between Sunni and Shia Muslims.
