Musnad Ahmad 32:587

'Abd al-Samad ibn 'Abd al-Warith narrated to us, my father narrated to us, Nafi' Abu Ghalib al-Bahili narrated to us, he witnessed Anas ibn Malik, who said: Al-'Ala' ibn Ziyad al-'Adawi said: "O Abu Hamzah, at the age of which men was the Prophet of Allah, may Allah bless him and grant him peace, when he was sent?" He said: "Forty years of age." He said: "Then what happened?" He said: "He was in Makkah for ten years, and in al-Madinah for ten years, so sixty years were completed for him, then Allah took him unto Himself." He said: "At the age of which men was he on that day?" He said: "Like the most vigorous of men, the best of them, the most beautiful of them, and the most full-fleshed of them." He said: "O Abu Hamzah, did you go on a military expedition with the Prophet of Allah, may Allah bless him and grant him peace?" He said: "Yes, I went on an expedition with him on the day of Hunayn. The polytheists came out in great numbers, and they charged at us until we saw our horses behind our backs. Among the polytheists was a man who was charging at us, crushing us and breaking us. When the Prophet of Allah, may Allah bless him and grant him peace, saw that, he dismounted, and Allah defeated them, so they turned back. The Prophet of Allah stood up when he saw the victory, and they began to be brought to him as captives, man by man, and they would pledge allegiance to him upon Islam. A man from the companions of the Messenger of Allah, may Allah bless him and grant him peace, said: 'I have a vow that if the man who was crushing us earlier is brought, I will surely strike his neck.' He said: The Prophet of Allah remained silent. The man was brought, and when he saw the Prophet of Allah, he said: 'O Prophet of Allah, I have repented to Allah; O Prophet of Allah, I have repented to Allah.' He said: The Prophet of Allah held back and did not accept his pledge of allegiance so that the other man could fulfill his vow. He said: The man began to look at the Prophet of Allah, may Allah bless him and grant him peace, for him to order him to kill him, but he was in awe of the Prophet of Allah, may Allah bless him and grant him peace, to kill him. When the Prophet of Allah, may Allah bless him and grant him peace, saw that he was not doing anything, he accepted his pledge of allegiance. He said: 'O Prophet of Allah, my vow!' He said: 'I did not hold back from him since earlier except for you to fulfill your vow.' He said: 'O Prophet of Allah, why did you not signal to me?' He said: 'It is not for a Prophet to signal.'"