Musnad Ahmad 1156:40

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