One division between Sunni and Shi'a Muslims is the question of who interprets the Qur'an and the Sunna. Who has that authority for the Shi'a?

Ali has that authority for the Shi' a

Explanation:

The division between Sunnis and Shi'as is the largest and oldest in the history of Islam.

Both Shi'as and Sunnis have good evidence to support their understanding of the succession. Sunnis argue that the Prophet chose Abu Bakr to lead the congregational prayers as he lay on his deathbed, thus suggesting that the Prophet was naming Abu Bakr as the next leader. The Shi'as' evidence is that Muhammad stood up in front of his Companions on the way back from his last Hajj, and proclaimed Ali the spiritual guide and master of all believers. Shi'a reports say he took Ali's hand and said that anyone who followed Muhammad should follow Ali.

Muslims who believe that Abu Bakr should have been the Prophet's successor have come to be known as Sunni Muslims. Those who believe Ali should have been the Prophet's successor are now known as Shi'a Muslims. It was only later that these terms came into use. Sunni means 'one who follows the Sunnah' (what the Prophet said, did, agreed to or condemned). Shi'a is a contraction of the phrase 'Shiat Ali', meaning 'partisans of Ali'.