Musnad Abu Ya'la al-Mawsili 154:152

Abu Khaythama narrated to us, Waki’ narrated to us, from Sufyan, from Firas, from Abu Salih, from Zadhan: That Ibn Umar slapped a slave of his, then he emancipated him. He then said, "I have no reward for this," and he picked up something from the ground. "I heard the Messenger of Allah (ﷺ) say: 'Whoever strikes his slave unjustly, there is no expiation for him except for emancipating him.'"