Musnad Ahmad 27:602

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Firas, I heard Dhakwan narrating from Zadhan, from Ibn Umar, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Whoever strikes a slave of his with a punishment for something he did not do, or slaps him, then his expiation is to set him free."