Musannaf Ibn Abi Shayba 28:723

Abu Mu'awiyah narrated to us, from al-A'mash, from 'Amr ibn Murrah, from Abu 'Ubaydah, from 'Abdullah, who said: When the day of Badr arrived, the Messenger of Allah (ﷺ) said: "What do you say regarding these captives?" Then he said: "None of them shall be killed except by ransom or by a strike to the neck."