The answer to the question above is how the leader should be chosen.
The main disagreement between Sunni and Shi'a Muslims during the rule of Ali was how the leader should be chosen. Mohammed never chose a successor for his leadership. Thus, the Muslims were divided into two sides; those who wanted Mohammed's son-in-law, Ali, for leadership, and those who wanted Mohammed's father-in-law and second-in-command, Abu Bakr, for the leadership.