Musannaf Ibn Abi Shayba 20:72

'Abdah narrated to us, from Sa'id, from Abu Ma'shar, from Ibrahim, regarding a man who acknowledges a child and then denies him: He said, "He performs li'an (imprecation) according to the Book of Allah, and the child is attached [to him] by the judgment of the Messenger of Allah, peace and blessings of Allah be upon him."