Musannaf Ibn Abi Shayba 28:733

Ibn Fudayl narrated to us, from Habib ibn Abi Amrah, from Mujahid, who said: The Messenger of Allah (may Allah bless him and grant him peace) consulted regarding the captives on the day of Badr. Abu Bakr said: "Your people, O Messenger of Allah, and your kin, your cousins; so take the ransom from them." Umar said: "Kill them." Then was revealed: "It is not for a prophet to have captives until he has thoroughly subdued the land." Mujahid said: "And the 'thorough subduing' (al-ithkhan) is the killing."