Musnad Abu Ya'la al-Mawsili 154:189

Zakariya ibn Yahya Zahmuwayh narrated to us, Ibrahim ibn Sa'id al-Madini Abu Ishaq narrated to us, he said: I heard Nafi' from Ibn Umar, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The woman in Ihram shall not wear the niqab (face veil)."