Musannaf Ibn Abi Shayba 38:102

Al-Fadl ibn Dukayn narrated to us, he said: Hasan ibn Salih narrated to us, he said: I heard Abdullah ibn al-Hasan mentioning from his mother that the Muslims killed Ubayd Allah ibn Umar on the day of Siffin. And the Muslims took his spoils, and it was wealth.