Ubayd bin Ti'la (may Allah have mercy on him) narrates: We went on a military campaign with Abdur-Rahman bin Khalid bin al-Walid, and four non-Arab disbelievers (captives) of the enemy were brought. He gave orders concerning them, and they were tied up and killed with arrows. This reached Abu Ayyub al-Ansari (may Allah be pleased with him), and he said: I heard the Messenger of Allah (peace and blessings be upon him) forbid killing a bound captive.
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 3987