Sayyiduna Abu Ayyub (may Allah be pleased with him) says: We went on a campaign with Abd al-Rahman bin Khalid bin al-Walid (may Allah be pleased with him) and camped in the land of the Romans. He says: Sayyiduna Abu Ayyub (may Allah be pleased with him) had made a mosque, and in the evenings we would go and sit in his gathering and benefit from his words. He says: One evening I was with him when a man came and said: Four Roman disbelievers (captives) have just been brought to the commander, and he ordered that they be tied up, and they were shot with arrows until they were killed. Sayyiduna Abu Ayyub (may Allah be pleased with him) stood up in alarm, came to Abd al-Rahman bin Khalid (may Allah be pleased with him) and said: Did you tie them up (to be killed)? I heard the Messenger of Allah (peace and blessings be upon him): he "forbade tying up an animal (to kill it)," and I would not like to have such-and-such (in return) for tying up even a single hen. So Abd al-Rahman bin Khalid (may Allah be pleased with him) freed four of his own slaves in their place. Abu Zur'ah (may Allah have mercy on him) says: Ubayd bin Ya'la was from Palestine; his home was in Ascalon.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السير / 18060