"Whoever beats his slave boy or slaps him for something that he has not done, then the expiation for that is to set him free." [Muslim]
عن ابن عمر -ما- أن النبي -صلى الله عليه وسلم- قال: «من ضرب غلاما له حدا لم يأته، أو لَطَمَه، فإن كفارته أن يعتقه». (مسلم)