Musnad Abu Ya'la al-Mawsili 3:218

Nasr ibn Ali al-Jahdhami narrated to us, my father narrated to me, from Ismail ibn Muslim, Abu Kathir, the freed slave of the Ansar, narrated to us, he said: I was with my master Ali ibn Abi Talib when he killed the people of al-Nahrawan. He said: It was as if the people felt something within themselves regarding their killing. Ali said: "O people, the Prophet of Allah (peace be upon him) told us about people who would pass through the religion just as an arrow passes through the game; they will not return to it until the arrow returns to its notch. The sign of that is that among them is a 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, and upon it are seven hairs. Seek him, for I see him among them." So they sought him and found him on the bank 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." The sign of that was that he was wearing an Arabian bow. He took it in his hand, then began to poke his deformed hand with it, saying: "Allahu Akbar, Allah and His Messenger have spoken the truth, 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.