Mukhtasar Muslim

It is narrated from Zadhan that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) called one of his slaves and saw a mark on his back, so he asked, "Did I cause you pain?" He replied, "No." Sayyiduna Ibn Umar (may Allah be pleased with them both) said, "You are free." Then he picked something up from the ground and said, "I did not even receive as much reward as this for freeing him. I heard the Messenger of Allah (peace and blessings be upon him) say: Whoever inflicts a legal punishment on his slave without a reason (i.e., beats him unjustly) or slaps him, its expiation (i.e., compensation, penalty) is that he should set him free."