Abu Dawud Book 27, Hadith Number 4107.
Chapter : Hanging down of lower garment by women.
Narated By Abdullah ibn Umar : The Apostle of Allah (PBUH) gave licence to others of the believers (i.e. the wives of the Prophet) to hang down their lower garment a span. Then they asked him to increase it, and he increased one span for them. They would send (the garment) to us and we would measure it one forearm’s length for them.