Sunni Islam is by far the most widespread form of Islam. It is the more orthodox of the two main branches of Islam but the origin of the Sunni-Shia schism was largely political in nature. The Sunnis accepted the first Caliph, Abu Bakr, whereas the Shiites rejected him in favor of Ali. Sunni Muslims hold strictly to the Sunnah - the sayings and actions of Muhammad - as collected in the Hadiths, which form the basis for Sunni jurisprudence. The formal head of Sunni Islam was the caliph. Some of its denominations were Ash'ari, Mu'tazila, Deobandi, Salafi/Wahhabism, and Sufism (the latter of which includes both Sunnis and Shi'ites and is nearly its own Islamic denomination in itself).