Hadith 10:266
Ibn ‘Umar told that he heard God’s Messenger say, “If anyone beats a servant for an offence he did not commit, or slaps him, the atonement due from him is to set him free.” Muslim transmitted it.
وَعَنِ ابْنِ عُمَرَ قَالَ: سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «مَنْ ضَرَبَ غُلَامًا لَهُ حَدًّا لَمْ يَأْتِهِ أَوْ لَطَمَهُ فَإِن كَفَّارَته أَن يعتقهُ» . رَوَاهُ مُسلم
Original Reference: Mishkat al-Masabih 3352