Musannaf Ibn Abi Shayba 9:754

Abu Bakr ibn Ayyash narrated to us, from Aslam al-Minqari, from Abdullah ibn Ubayd ibn Umayr, who said: Abdur Rahman ibn Awf sold a slave girl of his whom he used to have intercourse with, before he had established her innocence (from pregnancy). She then appeared pregnant with the one who bought her. He (the buyer) disputed with him before Umar. Umar said, "Did you have intercourse with her?" He said, "Yes." He said, "And you sold her before you established her innocence?" He said, "Yes." He said, "You were not worthy of that." So he summoned the *qafah* (experts in physiognomy/lineage), and they looked at him (the child) and attributed him to him (Abdur Rahman).