Musannaf Ibn Abi Shayba 13:574

Abu Bakr narrated to us, he said: Isa ibn Yunus and Abda ibn Sulayman narrated to us, from Uthman ibn Hakim, who said: I asked Salim ibn Abdullah, "May a man have intercourse with his *mudabbarah* (a slave woman promised freedom upon his death)?" He replied, "She is with me now."