Musannaf Ibn Abi Shayba 7:470

Ibn Idris narrated to us, from Husayn, from Hilal ibn Yasaf, who said: An old man was hasty and slapped a servant of his. Suwayd ibn Muqarrin said: "Are you unable to [control yourself] except against her face? I have seen myself as the seventh of seven brothers from the Banu Muqarrin, and we had no servant except one. The youngest of us slapped her, so the Messenger of Allah (peace and blessings of Allah be upon him) ordered us to free her."