Musnad Abu Ya'la al-Mawsili 209:13

Ibrahim ibn al-Hajjaj narrated to us, Hammad narrated to us, from Ayyub, from Nafi’, from Safiyyah bint Abi ‘Ubayd, from Umm Salamah: That when the Messenger of Allah (may Allah bless him and grant him peace) said what he said regarding the trailing of the garment (hem), she said: I said, "O Messenger of Allah, then what about us?" He said: "Let it trail by a span." She said: "Then the feet will be uncovered." He said: "Then let it trail by a cubit."