Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Safiyyah bint Abi Ubayd (may Allah be pleased with her) that when the Messenger of Allah (peace and blessings be upon him) mentioned the izar (lower garment), the wife of the Prophet (peace and blessings be upon him), Lady Umm Salamah (may Allah be pleased with her), said to him: "O Messenger of Allah! What about a woman?" The Prophet (peace and blessings be upon him) said: "She should let her izar hang down a handspan." Lady Umm Salamah (may Allah be pleased with her) said: "Then her feet will become uncovered!" The Prophet (peace and blessings be upon him) said: "Then she should let it hang down a forearm's length, but no more than that."