Musnad Ahmad 27:336

Waki’ narrated to us, from Sufyan, from Firas, from Abu Salih, from Zadhan, from Ibn Umar, that he called a servant of his and freed him. He then said: "I have no reward for this, nor for anything like it, that I picked up from the ground." I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: "Whoever slaps his servant, his expiation is to free him."