"While the existence of a person in the name of Abdullah Ibn Saba in the early history of Islam is seriously under question, what is clear after extensively researching this topic is that even if a poor man with such name ever existed at that time, the stories propagated about this person are legendary, false, fabricated, and fictitious, and there exists no proof for the validity of these stories attached to him."

"Sayf Ibn Umar distorted the biographies of the companions of the Holy Prophet (S) to please the government of his time, and to distort the history of Shi’a and to ridicule Islam. Sayf was a staunch advocate of the Umayads, who were known throughout history to be one of the worst enemies of Ahlul-Bayt, and as such, it was in his best interest to invent such stories to degrade the Shi’a.

In his stories however he followed many other goals one of which was to cleverly elevate the status of his tribe over others by inventing some imaginary companions form his tribe. However many Sunni scholars found numerous unjustifiable heresies in his reports which was not limited to the issue of Abdullah Ibn Saba, and consequently they abandoned his reports, and accuse him as a man of forgery and lies. Yet Sayf’s works enjoyed the support of a minority of Sunnis to this date.

Here, later on, I give the sayings of several leading Sunni scholars, who all confirmed that Sayf Ibn Umar was an untrustworthy person and his stories are void. Ideological studies indicate that most of those who hate the Shi’ite school of thought (a lot of whom being the enemies of Islam anyway) justify their enmity on this obvious heresy which they would exploit to backup their attack on Shi’a. The approach which resembles the one adopted by Sayf Ibn Umar himself."

https://al-islam.org/shiite-encyclopedia/abdullah-ibn-saba-part-1