Musnad Ahmad 5:688

Abu Nu'aym narrated to us, Isra'il narrated to us, from Ibrahim ibn 'Abd al-A'la, from Tariq ibn Ziyad, who said: 'Ali marched to Nahrawan and killed the Khawarij. He said: "Search, for the Prophet, peace and blessings of Allah be upon him, said: 'There will come a people who speak with the word of truth, but it will not pass beyond their throats. They will pass through Islam just as an arrow passes through the game. Their sign—or among them—is a black man with a deformed hand, in which there are black hairs. If he is among them, then you have killed the worst of people, and if he is not among them, then you have killed the best of people.'" He said: Then we found the one with the deformed hand. He said: So we fell down prostrating, and 'Ali fell down prostrating with us.