Musnad Ahmad 1156:164

Ya'la narrated to us, he said: Muhammad ibn Ishaq and Yazid narrated to us, he said: Muhammad ibn Ishaq informed us, from Nafi', from Safiyyah bint Abi 'Ubayd, from Umm Salamah, she said: I asked the Messenger of Allah, may Allah bless him and grant him peace, about the hems of women's garments. He said: "A span." I said: "Then their feet will be exposed, O Messenger of Allah." He said: "Then a cubit, do not exceed it."