(93) بَابُ مَنْ لَطَمَ عَبْدَهُ فَلْيُعْتِقْهُ مِنْ غَيْرِ إِيجَابٍ
Ibn 'Umar said, "I heard the Prophet, may Allah bless him and granthim peace, say, 'The expiation for someone who slaps his slave or beatshim more than he deserves is to set him free.'"
Reference: Al-Adab Al-Mufrad 177
In-book reference: Book 9, Hadith 22
English translation: Book 9, Hadith 177