Described as a mawla of the tribe of Khath'am, Musa was the son of Mus'ab ibn Rabi', a secretary to the last Umayyad caliph Marwan II who later submitted to the Abbasids during the Abbasid Revolution.[1] He himself appears to have enjoyed close relations with the Abbasid ruling family, having reportedly been a milk brother to the third caliph al-Mahdi at Humayma.[2]