Musnad Ahmad 1119:28

Al-Walid ibn Muslim narrated to us, he said: Safwan ibn ‘Amr narrated to me, from ‘Abd al-Rahman ibn Jubayr ibn Nufayr, from his father, from ‘Awf ibn Malik al-Ashja‘i, who said: I went out with those who went out with Zayd ibn Harithah from the Muslims in the Battle of Mu’tah. A reinforcement from Yemen accompanied me, who had nothing with him except his sword. A man from the Muslims slaughtered a camel, and the reinforcement asked him for a piece of its hide. He gave it to him, and he fashioned it into the shape of a shield. We proceeded, and we encountered the masses of the Romans, among whom was a man on a sorrel horse of his, wearing a gilded saddle and gilded armor. The Roman began to incite the Muslims, so the reinforcement sat in wait for him behind a rock. When the Roman passed by him, he hamstrung his horse, so it fell, and he climbed upon him and killed him, and he took possession of his horse and his weapons. When Allah granted victory to the Muslims, Khalid ibn al-Walid sent for him and took the spoils from him. ‘Awf said: I came to him and said: "O Khalid, do you not know that the Messenger of Allah (peace and blessings of Allah be upon him) decreed that the spoils belong to the killer?" He said: "Yes, but I considered it too much." I said: "You will return it to him, or I will certainly inform him about you in the presence of the Messenger of Allah (peace and blessings of Allah be upon him)." He refused to return it to him. ‘Awf said: We met in the presence of the Messenger of Allah (peace and blessings of Allah be upon him), and I narrated to him the story of the reinforcement and what Khalid had done. The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Khalid, what prompted you to do what you did?" He said: "O Messenger of Allah (peace and blessings of Allah be upon him), I considered it too much." The Messenger of Allah (peace and blessings of Allah be upon him) said: "O Khalid, return to him what you took from him." ‘Awf said: I said: "Take it, O Khalid, did I not fulfill my promise to you?" The Messenger of Allah (peace and blessings of Allah be upon him) said: "What is that?" I informed him, and the Messenger of Allah (peace and blessings of Allah be upon him) became angry and said: "O Khalid, do not return it to him. Will you not leave my commanders alone for me? You have the best of their affairs, and upon them is the burden of it." Al-Walid said: I asked Thawr about this hadith, and he narrated to me from Khalid ibn Ma‘dan, from Jubayr ibn Nufayr, from ‘Awf ibn Malik al-Ashja‘i, the like of it.