Musnad Ahmad 5:111

Abu Sa’id, the freed slave of Banu Hashim, narrated to us: Isma’il bin Muslim al-‘Abdi narrated to us: Abu Kathir, the freed slave of the Ansar, narrated to us, saying: I was with my master, ‘Ali bin Abi Talib, when the people of al-Nahrawan were killed. It was as if the people felt distressed within themselves regarding their killing. So ‘Ali said: "O people, the Messenger of Allah (peace and blessings of Allah be upon him) told us of people who would pass through the religion just as an arrow passes through the prey, then they would never return to it, until the arrow returns to its notch. The sign of that is that among them is a black man, with a deformed hand; one of his hands is like a woman’s breast, having a nipple like the nipple of a woman’s breast, with seven hairs around it. Seek him out, for I see him among them." They sought him out and found him at the edge of the river beneath the slain. They brought him out, and ‘Ali said: "Allahu Akbar (Allah is the Greatest). Allah and His Messenger have spoken the truth." He was wearing an Arabian bow of his, so he took it in his hand and began to poke his deformed hand with it, saying: "Allah and His Messenger have spoken the truth." The people said "Allahu Akbar" when they saw him and rejoiced, and what they had been feeling departed from them.