Musannaf Ibn Abi Shayba 19:174

Abu Bakr narrated to us, he said: Hafs narrated to us, from Hisham ibn Urwah, from his father, and from Ash'ath, from al-Hasan and Ibn Sirin, they said: There is no prescribed punishment (hadd) upon the one who falsely accuses (qadhif) an umm al-walad.