Musannaf Ibn Abi Shayba 19:176

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Mughira, from Ibrahim and al-Sha’bi, they both said: There is nothing [i.e., no prescribed punishment] upon the one who falsely accuses an *umm al-walad* [a slave woman who has borne a child to her master].