The succession to Muhammad is an extremely contentious issue. Muslims ultimately divided into two branches based on their political attitude towards this issue, which forms the primary theological barrier between the two major divisions of Muslims: Sunni and Shi'a, with the latter following Ali as the successor to Muhammad. The two groups also disagree on Ali's attitude towards Abu Bakr, and the two caliphs who succeeded him: Umar (or `Umar ibn al-Khattāb) and Uthman or (‘Uthmān ibn ‘Affān).