It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that he heard the Prophet (peace and blessings be upon him) say: Whoever punishes his slave for a crime he did not commit, or slaps him, its expiation is that he should set him free.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 5051