Musannaf Ibn Abi Shayba 9:1593

Al-Dahhak ibn Makhlad narrated to us, from Uthman ibn al-Aswad, from Ata, regarding a man who doubts his child: He said once, "Let him take an oath." And Umar ibn al-Khattab said, "Whoever acknowledges that he had intercourse with a slave-girl, we shall attach her child to him." And I asked Ikrimah ibn Khalid, and he said once, "Let him take an oath."