Arabic text
عن ابن عمر رضي الله عنهما أن النبي صلى الله عليه وسلم قال: «من ضرب غلاما له حدا لم يأته، أو لَطَمَه، فإن كفارته أن يعتقه».
Translation
Ibn ‘Umar (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Whoever beats his slave boy or slaps him for something that he has not done, then the expiation for that is to set him free."
Explanation
Whoever hits his slave boy while the boy did not do what deserves punishment, then the expiation for this sin is to manumit the slave boy.
Source: HadeethEnc.com