Musannaf Ibn Abi Shayba 28:612

Muhammad ibn Mus'ab narrated to us, from al-Awza'i, who said: I asked al-Zuhri about the enemy, if [the Muslims] prevail over them, should their *'uluj* (non-Arab/foreign combatants) be killed? He said: Umar used to kill the *'uluj* if he prevailed over them, and they would be taken captive along with that.