Musnad Ahmad 27:817

Waki’ narrated to us, from Sufyan, from Firas, from Abu Salih, from Zadhan, that Ibn Umar freed a slave of his and said: "I have no reward for him"—and he picked up something from the ground—"that weighs this, or the like of this. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'Whoever slaps his slave or strikes him, his expiation is to free him.'"