Yahya related to me from Malik from Abu Bakr ibn Nafi from hisfather Nafi, the mawla of Ibn Umar that Safiyya bint Abi Ubaydinformed him that Umm Salama, the wife of the Prophet, may Allah blesshim and grant him peace, said that when the lower garment of women wasmentioned to the Messenger of Allah, he said, "She lets it down ahandspan." Umm Salama said, "If it leaves her uncovered?" He said,"Then the length of a forearm and let her not increase it."
USC-MSA web (English) reference: Book 48, Hadith 13
Arabic reference: Book 48, Hadith 1667