Shia Islam primarily contrasts with Sunni Islam , whose adherents believe that Muhammad's father-in-law Abu Bakr was his proper successor. In it was estimated that perhaps 85 percent of Shias were Twelvers. Shia Islam is based on the Quran and the message of the Islamic prophet Muhammad attested in hadith recorded by the Shia, and certain books deemed sacred to the Shia Nahj al-Balagha. Shia consider Ali to have been divinely appointed as the successor to Muhammad, and as the first Imam.