Praise be to Allah.
Many of the classical and contemporary scholars are of the view that Shi‘ism or partisan support for ‘Ali (may Allah be pleased with him) began with the murder of ‘Uthmaan (may Allah be pleased with him), and that the one who planted the seed of Shi‘ism was the Jew ‘Abdullah ibn Saba’, at the end of ‘Uthmaan’s caliphate. This is something that is even admitted in the books of the Shi‘ah themselves.
See, for example: al-Maqaalaat wa’l-Firaq by al-Qummi (p. 20); Firaq ash-Shi‘ah by an-Nawbakhti (p. 22); Rijaal al-Kashshi (p. 108)
‘Abdullah ibn Saba’ was an extreme heretic and was the leader of the Saba’iyyah sect which said that ‘Ali (may Allah be pleased with him) was divine.
He was the first one to state that ‘Ali (may Allah be pleased with him) should have been the ruler on the basis of religious texts, and that he would return before the Day of Resurrection. He was also the first to openly cast aspersions upon the first three caliphs and the Sahaabah. All of these beliefs are fundamental to the view of the Raafidis.
‘Abdullah ibn Saba’ was a Jew who pretended to be Muslim. He was originally from Yemen, and travelled to spread his fitnah (turmoil) in the Hijaz, then Basra and Kufah. He went to Damascus during the caliphate of ‘Uthmaan ibn ‘Affaan (may Allah be pleased with him) and was expelled by its people, then he went to Egypt and started to openly promote his bid‘ah (innovation).
The scholars, in the past and more recently, transmitted reports of his fitnah and the efforts undertaken by him and his group to conspire and cause division among the Muslims. This is discussed in detail in the books that speak of sects, history and biography, by both Sunni and Shi‘i authors.
See, for example Maqaalaat al-Islamiyyeen by Abu al-Hasan al-Ash‘ari (1/32); al-Milal wa’n-Nihal by ash-Shahrastaani (1/174); Tareekh at-Tabari (4/340); al-Maqaalaat wa’l-Firaq by the Shi‘i al-Qummi (p. 20); Firaq ash-Shi‘ah by at-Nawbakhti (p. 22)
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: The first one to introduce the innovated view that ‘Ali was infallible and that he should have been the caliph on the basis of religious texts was the leader of these hypocrites, ‘Abdullah ibn Saba’, who was originally Jewish, then pretended to be Muslim and aimed to corrupt the Islamic religion as Paul had corrupted the Christian religion.
End quote from Majmoo‘ al-Fataawa (4/518)
The view that the Jew ‘Abdullah ibn Saba’ was the founder of the Raafidi sect, and that he established it in an effort to conspire against the Muslims and widen divisions among them, is a valid opinion that carries weight, with which the books of history and the study of sects and groups are filled, and it is not even denied by the Raafidis themselves.
With regard to the attempt of some of them to deny the existence of ‘Abdullah ibn Saba’, that is pure propaganda by means of which they are trying to refute what has become widely known among their opponents of the fact that this Jew was the founder of their madhhab.
The earlier scholars among both Sunnis and Shi‘ah alike were unanimously agreed that Ibn Saba’ was a real historical figure, so how can that which is agreed upon by both sides be denied?
For more information, please see the essay ‘Abdullah ibn Saba’ wa Atharuhu fi Ihdaath al-Fitnah fi Sadr al-Islam by Dr Sulayman ibn Hamad al-‘Awdah, which is one of the most important studies on this topic.
For more information, please see the book: Usool Madhhab ash-Shi‘ah al-Imaamiyyah al-Ithna ‘Ashariyyah: ‘Ard wa Naqd by Dr Naasir ibn ‘Abdullah al-Qifaari (1/71, 82)
And Allah knows best.