Musannaf Ibn Abi Shayba 38:159

Abu Mu'awiyah narrated to us, from al-A'mash, from Zayd ibn Wahb, from Ali, who said: When the day of al-Nahrawan arrived, he met the Khawarij, and they did not depart until they clashed with spears, and they were all killed. Ali said: "Search for Dhu al-Thudayyah." They searched for him but did not find him. Ali said: "I have not lied, nor have I been lied to; search for him." They searched for him and found him in a depression in the earth, with people from the slain upon him. Behold, there was a man, and upon his arm was something like the whiskers of a cat. Ali said: "Allahu Akbar," and the people said: "Allahu Akbar," and the people were amazed, and Ali was amazed.