Ibn 'Umar (May Allah be pleased with them) reported:
The Prophet (ﷺ) said, "The expiation for beating or slapping a slave on the face for something he has not done is to set him free."[Muslim]. (JA)
App reference (JA): Book 18, Hadith 95 (JA)
Arabic/English book reference (JA): Book 18, Hadith 1605 (JA)