Musannaf Ibn Abi Shaybah

Hazrat Yahya bin Kathir narrates that Abdullah bin Salam slapped one of his slaves. Upon this, he (Abdullah bin Salam) began to weep and said, "Take retribution from me." But the slave said, "O master! I will not take retribution from you." Then Ibn Salam said, "Allah Almighty will forgive every sin except for a slap on the face."