Abu Dawud Book 27, Hadith Number 4105.
Chapter : Hanging down of lower garment by women.
Narated By Umm Salamah, Ummul Mu’minin : Safiyyah, daughter of Abu Ubayd, said: When the Apostle of Allah (PBUH) mentioned lower garment, Umm Salamah, wife of the Apostle of Allah (PBUH), asked him: And a woman, Apostle of Allah? He replied: She may hang down a span. Umm Salamah said: Still it (foot) will be uncovered. He said: Then a forearm’s length, nor exceeding it.