Musannaf Ibn Abi Shaybah 35352 — Those who permitted killing children

Al-Awza'i (may Allah have mercy on him) said: I asked Imam al-Zuhri (may Allah have mercy on him) about the enemy: when they have been overpowered, are their 'uluj (non-Arab disbelieving men) to be killed as well? He said: Sayyiduna Umar (may Allah be pleased with him) used to kill the 'uluj when victory over them had been gained, and they would take captives along with that.