Is Ibn Saba a real person in History of Islam or just a myth?

I see sometimes his story is used as evidence . but as I know he is only mentioned in one book Tabari and the narrator of this story who Tabari said his name in his book is not known as a reliable source. Can some one mention some authentic evidences he was a real human and existed in history? also Tabari has said in his book Iwrote everything transmitted to