This lecture addresses a Prophetic tradition, ‘Who I was his master, ʿAlī is his master’ (man kuntu mawlāhu fa-ʿAlī mawlāhu). This hadith is cited by Shīʿī and Sunnī Muslims alike to articulate ʿAlī’s exclusive legitimacy to rule or his privileged standing in the first Muslim community; however, its origins remain understudied. The lecturer will present the results of an analysis of this Prophetic hadith through the isnād-cum-matn method, which not only pinpoint the time of its circulation or creation, but also identify the key transmitters responsible for the circulation of its variants. The sectarian tendencies of the pivotal figures in the dissemination of this hadith as depicted in early Arabic biography also provide valuable insights which can help to navigate the ambiguous sectarian boundaries in early Islam.