Musnad Abu Ya'la al-Mawsili 209:99

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Muhammad bin Ishaq narrated to us, from Nafi’, from Safiyyah bint Abi ‘Ubayd, from Umm Salamah, who said: The Messenger of Allah (ﷺ) said: "The train (hem) of women [should be] a span." I said: "O Messenger of Allah, then their feet will be exposed." He said: "Then a forearm, and they shall not exceed it."