Al-Adab Al-Mufrad, Book 9, Hadith 180 (English) ⓘ

Abu 'Umar Zadhan said, "We were with Ibn 'Umar when he summoned a slave of his whom he had beaten and he uncovered his back. 'Does it hurt?' he asked. 'No,' he replied. Then he set him free. He picked up a stick from the ground and then said, 'I do not have a reward (for him) worth as much as this stick.' I asked, 'Abu 'Abdu'r-Rahman, why do you say this?' He replied, 'I heard the Prophet, may Allah bless him and grant him peace, say, "The expiation of someone who beats a slave more than he deserves or slaps his face is that he must set him free."'"

حَدَّثَنَا مُوسَى، قَالَ‏:‏ حَدَّثَنَا أَبُو عَوَانَةَ، قَالَ‏:‏ حَدَّثَنَا فِرَاسٌ، عَنْ أَبِي صَالِحٍ، عَنْ زَاذَانَ أَبِي عُمَرَ قَالَ‏:‏ كُنَّا عِنْدَ ابْنِ عُمَرَ، فَدَعَا بِغُلاَمٍ لَهُ كَانَ ضَرَبَهُ فَكَشَفَ عَنْ ظَهْرِهِ فَقَالَ‏:‏ أَيُوجِعُكَ‏؟‏ قَالَ‏:‏ لاَ‏.‏ فَأَعْتَقَهُ، ثُمَّ رَفَعَ عُودًا مِنَ الأَرْضِ فَقَالَ‏:‏ مَالِي فِيهِ مِنَ الأَجْرِ مَا يَزِنُ هَذَا الْعُودَ، فَقُلْتُ‏:‏ يَا أَبَا عَبْدِ الرَّحْمَنِ، لِمَ تَقُولُ هَذَا‏؟‏ قَالَ‏:‏ سَمِعْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ أَوْ قَالَ‏:‏ مَنْ ضَرَبَ مَمْلُوكَهُ حَدًّا لَمْ يَأْتِهِ، أَوْ لَطَمَ وَجْهَهُ، فَكَفَّارَتُهُ أَنْ يُعْتِقَهُ‏.‏