Sahih Muslim Book - 27, Hadith - 4299

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 لِى فِيهِ مِنَ الأَجْرِ مَا يَزِنُ هَذَا إِنِّى سَمِعْتُ رَسُولَ اللَّهِ -صلى الله عليه وسلم- يَقُولُ « مَنْ ضَرَبَ غُلاَمًا لَهُ حَدًّا لَمْ يَأْتِهِ أَوْ لَطَمَهُ فَإِنَّ كَفَّارَتَهُ أَنْ يُعْتِقَهُ ».

It was narrated from Zadan that Ibn 'Umar called a slave of his and he saw marks on his back. He said to him: "Have I caused you pain?" He said: "No." He said: "You are free." Then he picked up something from the ground and said: "I will not have any reward for it, not even the weight of this. I heard the Messenger of Allah (s.a.w) say: 'Whoever beats a slave without him having done anything to deserve it, or slaps him, his expiation is to manumit him."'