Musnad Ahmad 1156:209

Muhammad ibn Ubayd narrated to us, he said: Ubaydullah narrated to us, from Nafi’, from Sulayman ibn Yasar, from Umm Salamah, she said: I said: "O Messenger of Allah, what about the women?" He said: "They should let it down by a span." I said: "Then their [feet] will be uncovered, O Messenger of Allah." He said: "Then a forearm’s length, and they should not exceed that."