Sahih Muslim 1657 b
Zadhan reported that Ibn Umar called his slave and he found the marks (of beating) upon his back. He said to him:
I have caused you pain. He said: No. But he (Ibn Umar) said: You are free. He then took hold of something from the earth and said: There is no reward for me even to the weight equal to it. I heard Allah's Messenger (ﷺ) as saying: He who beats a slave without cognizable offence of his or slaps him (without any serious fault), then expiation for it is that he should set him free.
وَحَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ - وَاللَّفْظُ لاِبْنِ الْمُثَنَّى - قَالاَ حَدَّثَنَا مُحَمَّدُ، بْنُ جَعْفَرٍ حَدَّثَنَا شُعْبَةُ، عَنْ فِرَاسٍ، قَالَ سَمِعْتُ ذَكْوَانَ، يُحَدِّثُ عَنْ زَاذَانَ، أَنَّ ابْنَ عُمَرَ، دَعَا بِغُلاَمٍ لَهُ فَرَأَى بِظَهْرِهِ أَثَرًا فَقَالَ لَهُ أَوْجَعْتُكَ قَالَ لاَ ‏.‏ قَالَ فَأَنْتَ عَتِيقٌ ‏.‏ قَالَ ثُمَّ أَخَذَ شَيْئًا مِنَ الأَرْضِ فَقَالَ مَا لِي فِيهِ مِنَ الأَجْرِ مَا يَزِنُ هَذَا إِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏
"‏ مَنْ ضَرَبَ غُلاَمًا لَهُ حَدًّا لَمْ يَأْتِهِ أَوْ لَطَمَهُ فَإِنَّ كَفَّارَتَهُ أَنْ يُعْتِقَهُ ‏"‏ ‏.‏