Musannaf Ibn Abi Shayba 27:708

Ibn ‘Uyaynah narrated to us, from ‘Amr, from al-Hasan ibn Muhammad, from ‘Ubayd Allah ibn Abi Rafi’, who informed him, from ‘Ali, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "What makes you know? Perhaps Allah looked upon the people of Badr and said: 'Do what you wish, for I have already forgiven you.'"