Musannaf Ibn Abi Shayba 13:565

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from Ibn Jurayj, who said: I asked 'Ata': "Did Ibn 'Umar have sexual intercourse with his *mudabbarah* (a slave woman promised freedom upon her master's death)?" He said: "Yes, and Ibn 'Abbas [did as well]."