After Ja'far al-Sadiq's death during the reign of the Abbāsids, various Shīī groups organised in secret opposition to their rule. Among them were the supporters of the proto-Ismāīlī community, of whom the most prominent group were called the "Mubārakiyyah".
There are hadīth which state that Ismāīl ibn Jafar "al-Mubārak"[citation needed] would be heir to the Imamate, as well as those that state Musa al-Kadhim[3][23] was to be the heir. However, Ismāīl predeceased his father.
Some of the Shīah claimed Ismāīl had not died, but rather gone into hiding, but the proto-Ismāīlī group accepted his death and therefore that his eldest son, Muḥammad ibn Ismāīl, was now Imām. Muḥammad remained in contact with this "Mubārakiyyah" group, most of whom resided in Kūfah.[citation needed]
In contrast, Twelvers don't believe that Isma'il ibn Jafar was ever given the nass ("designation of the Imamate"),[24][25] but they acknowledge that this was the popular belief among the people at the time.[26] Both Shaykh Tusi[24] and Shaykh al-Sadūq[25] did not believe that the divine designation was changed (called Bada'), arguing that if matters as important as Imāmate were subject to change, then the fundamentals of belief should also be subject to change. Thus Twelvers accept that Mūsá al-Kāżim was the only son who was ever designated for Imāmate.
this is also from Wikipedia, i am not able to scans any books but maybe one day.
