Shia Islam and Sunnism split in the aftermath of the death of Muhammad based on the politics of the early caliphs. Due to the Shi'a belief that Ali should have been the first caliph, the three caliphs that preceded him, Abu Bakr, Umar, and Usman, were considered illegitimate usurpers. Because of this, any hadith that were narrated by these three caliphs (or any of their supporters) were not accepted by Shi'a hadith collectors.