Musnad Ahmad 5:115

Abu Sa’id narrated to us, Isra’il narrated to us, from Abu Ishaq, from Harithah ibn Mudarrib, from ‘Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) said on the day of Badr: "Whoever you are able to capture from the Banu ‘Abd al-Muttalib, [do not kill them], for they have come out against their will."