Musannaf Ibn Abi Shayba 26:314

Waki’ narrated to us, he said: Sufyan narrated to us, from al-Harish al-Bajali, from his father, that a man and his son—or two brothers—were killed on the day of Siffin together, and it was not known which of them was killed first. He said: So Ali made each of them inherit from the other.