Musannaf Ibn Abi Shayba 19:178

Abu Bakr narrated to us, he said: Abbad narrated to us, from Ash'ath, from al-Hasan and Muhammad, they both said: There is no *hadd* (prescribed punishment) upon the one who falsely accuses an *umm al-walad* (a slave woman who has borne a child for her master) of unchastity.