Musnad Ahmad 8:117

Abu Mu'awiyah narrated to us, Abu Ishaq al-Shaybani narrated to us, from Muhammad ibn Ubaydullah al-Thaqafi, from Sa'd ibn Abi Waqqas, who said: When the day of Badr arrived, my brother Umayr was killed, and I killed Sa'id ibn al-'As, and I took his sword, which was called Dhu al-Katifah. I brought it to the Prophet of Allah, peace and blessings of Allah be upon him. He said: "Go and throw it into the collection (al-qabd)." I returned, and there was within me what only Allah knows regarding the killing of my brother and the taking of my spoils. I had not gone but a short distance when Surat al-Anfal was revealed. The Messenger of Allah, peace and blessings of Allah be upon him, said to me: "Go and take your sword."