Musnad Ahmad 32:1611

'Ali ibn 'Asim narrated to us, from Humayd, from Anas, and he mentioned a man, from al-Hasan, who said: The Messenger of Allah (may Allah bless him and grant him peace) consulted the people regarding the captives on the day of Badr. He said: "Indeed, Allah has given you power over them." He said: So 'Umar ibn al-Khattab stood up and said: "O Messenger of Allah, strike their necks [execute them]." He said: So the Prophet (may Allah bless him and grant him peace) turned away from him. He said: Then the Messenger of Allah (may Allah bless him and grant him peace) returned and said: "O people, indeed Allah has given you power over them, and they were only your brothers yesterday." He said: So 'Umar stood up and said: "O Messenger of Allah, strike their necks." He said: So the Prophet (may Allah bless him and grant him peace) turned away from him. He said: Then the Prophet (may Allah bless him and grant him peace) returned and said the same thing to the people. So Abu Bakr stood up and said: "O Messenger of Allah, we see that you should pardon them and accept ransom from them." He said: So the distress that was upon the face of the Messenger of Allah (may Allah bless him and grant him peace) departed. He said: So he pardoned them and accepted the ransom from them. He said: And Allah revealed: "Had it not been for a decree from Allah that preceded, you would have been touched by a great punishment for what you took." [Al-Anfal: 68]