Musnad Ahmad 1090:2

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Husayn, who said: I heard Hilal ibn Yasaf narrating from Suwayd ibn Muqarrin, who said: We were selling cloth in the house of Suwayd ibn Muqarrin. He said: A slave girl of Suwayd came out and spoke to a man among us, and he insulted her, so he slapped her face. Suwayd said: You slapped her? I have seen myself as the seventh of seven brothers, and we had no servant but her. One of us went and slapped her, so the Messenger of Allah (peace and blessings of Allah be upon him) commanded us to free her.