What issue divided Shiite and Sunni Muslims?

After Muhammad's death, Muslims needed someone to rule according to their holy book, the Qu'ran. It divided the Muslims because of their disagreement on who they thought would be an ideal leader (caliph). Sunni's believed that any Muslim most capable of ruling should be able to rule, but the Shia believed that a Muslim leader would need to be a descendant from Muhammad and related in some way.