Musannaf Ibn Abi Shayba 3:3865

Hushaym narrated to us, from Khalid, from Abu Qilabah, who said: Umar ibn al-Khattab would not allow a slave woman to wear a head covering (niqab/khimar) during his caliphate. He said: Umar said, "The head covering is only for free women, so that they are not bothered."