Musnad Ahmad #1189

It was narrated that Tariq bin Ziyad said: ‘Ali marched to an-Nahrawan and killed the Khawarij. Then he said: Look, for the Prophet (ﷺ) said: `There will come a people who will speak the word of truth but it will not go any further than their throats; they will pass out of Islam as the arrow passes out of the prey. Their sign - or among them - will be a black man with a deformed arm, on which there are black hairs. If he is among them, then you will have killed the worst of people; if he is not among them, then you will have killed the best of people.” Then we found the one with the deformity, and we fell down in prostration and ‘Ali fell down in prostration with us. .