Similar to other schools of thought in Islam, Shia Islam is based on the teachings of the Islamic holy book, the Qurʻān and the message of the final prophet of Islam, Muhammad. In contrast to other schools of thought, Shia Islam holds that Muhammad's family, the Ahl al-Bayt ("the People of the House"), and certain individuals among his descendants, who are known as Imams, have special spiritual and political authority over the community. Shia Muslims further believe that Ali, Muhammad's cousin and son-in-law, was the first of these Imams and was the rightful successor to Muhammad and thus reject the legitimacy of the first three caliphs.
Shias regard Ali as the second most important figure after Prophet Muhammad. According to them, Muhammad suggested on various occasions during his lifetime that Ali should be the leader of Muslims after his demise. According to this view, Ali as the successor of Muhammad not only ruled over the community in justice, but also interpreted the Sharia Law and its esoteric meaning. Hence he was regarded as being free from error and sin (infallible), and appointed by God by divine decree (Nass) to be the first Imam. Ali is known as "perfect man" (al-insan al-kamil) similar to Muhammad according to Shia viewpoint. As a result, Shias use Hadiths attributed to Muhammad and Imāms, and credited to the Prophet's family and close associates, in contrast to the Sunni traditions where the sunnah is largely narrated by companions.Subsequently, the hadith contrasts between the Shias and Sunnis are one of the main reasons for friction between them, as Sunnis do not accept Shia hadith and vice versa.