Could you please clarify to me why honorable Ali ibn Abu Talib (ra) was called by the title "Al Murtaza"?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Al-Khaadimi clarified in his book Bareeqah Mahmoodiyyah the reason why ‘Ali was called Al-Murtadha. He said, “This is because the Prophet had chosen him to replace him among his family in Al-Madeenah in the battle of Tabook, and he told him: “You are to me like Haaroon (Aaron) to Moosa (Moses) except that there is no Prophet after me.” [Al-Bukhari] Or this is because the Prophet was pleased with his actions, or due to his brotherhood and companionship of the Prophet, sallallaahu ‘alayhi wa sallam, and his virtues .”

Allaah Knows best.