Musannaf Ibn Abi Shayba 9:328

Abu Mu'awiyah narrated to us, from al-Hasan ibn 'Amr, from al-Sha'bi, who said: Masruq wrote to his family: "Look into my slave girl, and do not seek her (for marriage), for I have not obtained from her anything except that which makes her forbidden to my children: touching and looking."