Al-Adab Al-Mufrad #180

Abu 'Umar Zadhan said, "We were with Ibn 'Umar when he summoned a slave of his whom he had beaten and he uncovered his back. 'Does it hurt?' he asked. 'No,' he replied. Then he set him free. He picked up a stick from the ground and then said, 'I do not have a reward (for him) worth as much as this stick.' I asked, 'Abu 'Abdu'r-Rahman, why do you say this?' He replied, 'I heard the Prophet, may Allah bless him and grant him peace, say, "The expiation of someone who beats a slave more than he deserves or slaps his face is that he must set him free."'"