Musnad Ahmad 27:818

'Abd al-Rahman narrated to us, from Sufyan, from Firas, Abu Salih informed me, from Zadhan, who said: I was with Ibn 'Umar when he called a slave of his and emancipated him. Then he said: "I have no reward in him equal to this—or weighing this—for I heard the Messenger of Allah (ﷺ) say: 'Whoever strikes a slave of his with a punishment he did not commit, or wrongs him—or slaps him, 'Abd al-Rahman was uncertain—then his expiation is to emancipate him.'"