Musnad Abu Ya'la al-Mawsili 3:216

Ubayd Allah ibn Umar al-Qawariri narrated to us, Abd al-Rahman ibn al-Aryan al-Harithi narrated to us, al-Azraq ibn Qays narrated to us, from a man from Abd al-Qays, who said: I witnessed Ali on the day he killed the people of al-Nahrawan. He said: Ali said when they were killed, "Search for Dhu al-Thudayyah (the one with the breast) or al-Mukhdaj (the malformed one)." He mentioned something about that which I do not remember. He said: So they searched for him, and behold, they found an Abyssinian [man] like a camel, on his shoulder was something like a woman’s breast. Abd al-Rahman said: I think he said [it had] hair. If a human’s soul were to depart out of joy, Ali’s soul would have departed that day. He said: "Allah and His Messenger spoke the truth. Whoever among the people tells me that he saw him before this killing of his, then I am a liar."