Musannaf Ibn Abi Shayba 19:349

Abu Bakr narrated to us, he said: Hafs narrated to us, from Ubayd Allah, from Nafi’, from Ibn Umar, that Umar was brought slave-girls from the slave-girls of the government (imarah), whom some of the boys (ghilman) of the government had forced. So he struck the boys, and he did not strike the slave-girls.