Musannaf Ibn Abi Shayba 18:1280

Abu Bakr narrated to us, he said: Yahya bin Abd al-Malik bin Abi Ghaniyah narrated to us, from his father, from al-Hakam: That al-Abbas bin Abd al-Muttalib slapped a man, so the Prophet, peace and blessings of Allah be upon him, allowed the man to exact retribution from al-Abbas, then he (the man) pardoned him.