Zadhan said: I came to Ibn ‘Umar when he set his slave free. He took a stick or something else from the earth and said; for me there is no reward even equivalent to this. I heard the Messenger of Allah (May peace be upon him) say: If anyone slaps or beats his slave the atonement due from him is to set him free.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، وَأَبُو كَامِلٍ قَالاَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ فِرَاسٍ، عَنْ أَبِي صَالِحٍ، ذَكْوَانَ عَنْ زَاذَانَ، قَالَ أَتَيْتُ ابْنَ عُمَرَ وَقَدْ أَعْتَقَ مَمْلُوكًا لَهُ فَأَخَذَ مِنَ الأَرْضِ عُودًا أَوْ شَيْئًا فَقَالَ مَا لِي فِيهِ مِنَ الأَجْرِ مَا يَسْوَى هَذَا سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " مَنْ لَطَمَ مَمْلُوكَهُ أَوْ ضَرَبَهُ فَكَفَّارَتُهُ أَنْ يُعْتِقَهُ " .
sunnah.com