Musnad Ahmad 5:285

Al-Walid ibn al-Qasim ibn al-Walid al-Hamdani narrated to us, Isra'il narrated to us, Ibrahim—meaning Ibn 'Abd al-A'la—narrated to us, from Tariq ibn Ziyad, who said: We went out with 'Ali to the Khawarij, and he killed them. Then he said: "Look, for the Prophet of Allah (peace and blessings of Allah be upon him) said: 'There will emerge a people who speak the truth, but it will not pass beyond their throats. They will exit from the truth just as an arrow exits from the game. Their sign is that among them is a black man whose arm is deformed (mukhdaj), and in his hand are black hairs.' If it is him, then you have killed the worst of people, and if it is not him, then you have killed the best of people." So we wept. Then he said: "Search." We searched and found the deformed man. We fell down in prostration, and 'Ali fell down in prostration with us. Except that he said: "They speak the word of truth."