Abdullah Ibn Saba'
Abd Allah ibn Sabaʾ al-Ḥimyarī (or "Sabāʾ", also sometimes called ibn al-Sawdāʾ, ibn Wahb, or ibn Ḥarb) was a 7th-century Jew and a figure in Islamic history who may have had an actual historical existence and often associated with a group of followers called the Sabaʾiyya.
Some modern historical views are not clear what person lay behind this figure. Some believe that Abdullah Ibn Saba may have been actually several figures(e.g. Hodgson), semi legendary (Caetani, Momen Moojan), or legendary and fictional (Taha Hussein, Ali al-Wardi, Bernard Lewis, Wilferd Madelung, Askari) but the Jewish rabbi and biblical scholar Israel Friedlander and Sabatino Moscati affirm his existence. His Jewish origin has been contested. Except Taha Hussein and inspired by theological and communal sentiments rather than by logic, most modern Sunni writers either repeat or reinterpret the ideas ascribed to Ibn Saba' in later sources. In a similar vain, all Shia writers argue that Ibn Saba' is not a historical figure to rid Shi'aism of the old accusation by their adversaries that it was originally based on Judaic doctrines. Modern Muslim writers also tend to discredit Tabari's account of Ibn Saba as "sheer fiction". Regarding his importance in killing of Othman and introducing Shi'a is disputed as surer sources other than Tabari exclude him from any major role. Modern historians assert that Sayf fabricated the episode about killing of Uthman to "exonerate the people of Medina from participation in the caliph's murder".
According to Tabari, based on traditions collected by Sayf ibn Umar, Ibn Saba' was a Yemenite Jew who embraced Islam. During the time of Ali ibn Abi Taleb, he introduced a number of concepts that later were ascribed to more extreme factions of Shia Islam, or ghulat. According to these traditions, the exaltation of Ali, his divine appointment by the Islamic prophet Muhammad as a successor, the concept of ghayba and return (rajʿa) were first formulated and expressed by Ibn Sabaʾ and his followers (the Sabaʾiyya). He and his followers are sometimes said to be the ones who enticed the Egyptians against Uthman on the ground of Ali's special right of succession, and participated in further instigation at later conflicts.
In Shia' views, the claim that Ibn Saba' as a convert Jew is the founder of Shia is propaganda. Although the existence of Abdullah Ibn Saba' is seriously under question, even if such a person existed, the stories propagated about this person are legendary, false, fabricated, and fictitious. In traditional Shi'a sources, he is sometimes viewed as an extremist Shia (ghulat), himself cursed by Jaʿfar al-Ṣādiq. Nevertheless, Ibn Sabaʾ became the subject of a tradition used by different Shia factions to both attack and defend extreme Shia groups. According to this tradition, and depending on the different interpretations, Ali either burned or exiled him and his followers for declaring Ali as God.
Read more about Abdullah Ibn Saba': Shī‘a Views, Sayf Ibn Umar and Other Sources On Ibn Saba