Musannaf Ibn Abi Shayba 9:1671

Waki’ narrated to us, from Sufyan, from Hammad and al-Hakam, who both said: "If he acknowledges his child, then denies him..." Al-Hakam said: "He is to be lashed." Hammad said: "The child is binding upon him by the acknowledgment, and he must perform *li’an* (imprecation)." And he mentioned that Sufyan used to say this.