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.
مَا لِي فِيهِ مِنَ الأَجْرِ مَا يَسْوَى هَذَا سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ مَنْ لَطَمَ مَمْلُوكَهُ أَوْ ضَرَبَهُ فَكَفَّارَتُهُ أَنْ يُعْتِقَهُ