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]. (PT)
App reference (PT): Book 18, Hadith 95 (PT)
Arabic/English book reference (PT): Book 18, Hadith 1605 (PT)