Musannaf Ibn Abi Shayba 19:298

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Isma'il ibn Salim, that there were two men between whom there was a dispute. One of them said: "There is no child born in Kufa who is a child of adultery except that the other [man] resembles him." The other said: "If what is with the other were uncovered, it would be an immoral act, except that I would recognize it." So Al-Sha'bi was asked about that, and he said: "There is no prescribed punishment (*hadd*) upon either of them."