Imam Malik's Muwatta, Chapter: Dress

Yahya related to me from Malik from Abu Bakr ibn Nafi from his father Nafi, the mawla of Ibn Umar that Safiyya bint Abi Ubayd informed him that Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, said that when the lower garment of women was mentioned to the Messenger of Allah, he said, "She lets it down a handspan." Umm Salama said, "If it leaves her uncovered?" He said, "Then the length of a forearm and let her not increase it."