Musannaf Ibn Abi Shayba 16:660

Abu Bakr narrated to us, he said: Mu'tamir ibn Sulayman narrated to us, from Ubayd Allah ibn Umar, from Nafi', from Sulayman ibn Yasar, from Umm Salamah, who said: The Prophet, peace and blessings of Allah be upon him, was asked: "How much should a woman let her garment trail?" He said: "A span." It was said: "Then her [feet] will be uncovered." He said: "A cubit, and do not exceed it."