Musannaf Ibn Abi Shayba 26:297

Waki’ narrated to us, he said: Sufyan narrated to us, from Dawud ibn Abi Hind, from Abdullah ibn Ubayd ibn Umayr, he said: I wrote to a brother of mine among the Banu Zurayq: "To whom did the Messenger of Allah (peace be upon him) adjudge the son of the woman subject to li'an (imprecation)?" He wrote back to me: "The Messenger of Allah (peace be upon him) adjudged him to his mother; she is in the position of his father and in the position of his mother."