Musannaf Ibn Abi Shayba 19:182

Abu Bakr narrated to us, he said: Abd al-Wahhab ibn 'Ata' narrated to us, from Sa'id, from Abu Yazid al-Madini, that Umar ibn Abd al-Aziz flogged a man who had slandered the *umm walad* (a slave woman who has borne a child to her master) of a man, even though she had not been emancipated.