Musnad Ahmad 1156:61

Yazid narrated to us, he said: Muhammad ibn Ishaq narrated to us, from Nafi’, from Safiyyah bint Abi ‘Ubayd, from Umm Salamah, the wife of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The hems of women [should be] a span." I said: "Then their feet will be exposed, O Messenger of Allah." He said: "Then a forearm, and do not exceed it."