Musnad Ahmad 1090:1

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Salamah—meaning Ibn Kuhayl—from Mu'awiyah ibn Suwayd, who said: I slapped a freed slave of ours, so my father said to him: "Retaliate." Then he said: We, the clan of Banu Muqarrin, were seven, and we had no servant except one woman, and one of us slapped her. The Prophet, peace and blessings of Allah be upon him, said: "Free her." It was said to him: "They have no servant other than her." He said: "Let her serve them, and when they have no need of her, let them free her."