Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) said: We were sitting with the Messenger of Allah (peace and blessings be upon him) when a man with his face covered passed by us, and the Messenger of Allah (peace and blessings be upon him) said: "There will be division and discord among the people, and this man and his companions will be upon the truth." Ka'b (may Allah be pleased with him) said: I caught up with him and looked at him closely until I recognised him. We used to ask Ka'b: Who was that man? But he would refuse to tell us, until Ka'b went with Ali (may Allah be pleased with him) to Kufah and remained there until he died; so it was as if we came to know that the man was Sayyiduna Ali (may Allah be pleased with him).
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16372