 Complete the following phrase:  Ever since the martyrdom of Ali, fourth caliph among the Khulafa Rashidun and the son-in-law of Muhammad, in 661, which led to the establishment of the Umayyad Caliphate, a small part of the Muslim community rejected the Umayyads as usurpers and called for the establishment of a regime led by a member of the Ahl al-Bayt, the Family of Muhammad. The Abbasids, who claimed descent from Muhammads paternal uncle Abbas ibn Abd al-Muttalib, profited from this during their 