Muhammad bin Ka'b and Uthman bin Yahudha narrated from men of their people, who said: He then mentioned the story of the Trench and how Ali bin Abi Talib (may Allah be pleased with him) killed Amr bin Abd Wudd. Then Ali (may Allah be pleased with him) came towards the Messenger of Allah (peace and blessings be upon him) with his face shining. Sayyiduna Umar (may Allah be pleased with him) said: Why did you not kill him and take his coat of mail? It was the finest coat of mail among the Arabs. Sayyiduna Ali (may Allah be pleased with him) said: I struck him, and he shrank back in fear towards his ranks, and I felt ashamed to take his coat of mail.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12771