The eldest son of Ali b. Abi Talib, Muhammad's cousin, and Fatima, Muhammad's daughter.
Hasan took part in the campaigns led by his father during his caliphate. After his father's death in 661, Hasan was nominated as caliph. However, in Syria the governor Muawiya had already been declared caliph, and he determined not to recognize Hasan's claim. The two claimants prepared for battle, but Muawiya's superior forces probably persuaded Hasan to refrain from fighting and ultimately abdicate, with the promise of accession after Muawiya's death. Hasan lived the remainder of his life quietly. He died around 670, either from a prolonged illness or poisoning at the hand of his wife at the instigation of Muawiya. According to Shi`ites Hasan is the second rightful leader of the Muslim community (Imam) after his father.