The Book of Virtue, Good Manners and Joining of the Ties of Relationship (Kitab Al-Birr was-Salat-I-wa'l-Adab)
Muslim :: Book 32 : Hadith 6324
Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: When any one of you fights with his brother, he should not slap at the face.