Musnad Ahmad 1090:3

Hushaym narrated to us, Husayn informed us, from Hilal ibn Yasaf, that a man was staying in the house of Suwayd ibn Muqarrin. He said: He slapped a servant. Suwayd became angry and said: "Did you find nothing but the center of his face? I have seen myself, and we were seven brothers from the children of Muqarrin, and we had no servant except one. One of us went to him and slapped him. The Messenger of Allah (peace and blessings of Allah be upon him) commanded us, when we returned, to set him free, so we set him free."