Treaty of Hudaybiyyah
A large group of Ali's army were discontented with the conclusion of that arbitration, and broke off into a separate group that became known as the Khawarij or Kharijites. Ibn Abbas played a key role in convincing a large number of them to return to Ali; 20,000 of 24,000 according to some sources. He did so using his knowledge of Muhammad's biography, in particular, the events of the Treaty of Hudaybiyyah.