The Ismailis and the Twelvers split over the succession to Imam Ja'far al-Sadiq. Ismailis contend that Jafar had designated his son Isma'il ibn Jafar as his heir and the next Imam in the hereditary line, and thus the Isma'ilis follow the Imamate of Isma'il and his progeny. Although Imam Ismail predeceased his father, Isma'il supposedly had designated his own son Muhammad ibn Ismail as the next hereditary Imam to follow. In opposition to this belief, the Twelvers believe that Imam Ismail's younger brother Musa al Kadhim was, from the beginning, the rightful successor to Imam Jafar and that Ismail was never a contender.