Musannaf Ibn Abi Shayba 36:152

'Abd al-Rahim ibn Sulayman narrated to us, from Ash'ath, from Ibn Sirin, from 'Ubaydah al-Salmani, who said: Seventy men from the polytheists were taken captive on the day of Badr, and seventy of them were killed. The Messenger of Allah (may Allah bless him and grant him peace) gathered the Ansar and gave them a choice. He said: "Whatever you wish; if you wish, kill them, and an equal number of you will be killed. And if you wish, take their ransom and strengthen yourselves with it in the way of Allah." They said: "O Messenger of Allah, we will take the ransom to strengthen ourselves with it in the way of Allah, and an equal number of us will be killed." He said: So an equal number of them were killed on the day of Uhud.