Yahya related to me from Malik from Muhammad ibn Zayd ibn Qunfudhthat his mother asked Umm Salama, the wife of the Prophet, may Allahbless him and grant him peace, "What clothes can a woman wear inprayer?" She said, "She can pray in a shift that reaches down andcovers the top of her feet."
App reference: Book 8, Hadith 37
USC-MSA web (English) reference: Book 8, Hadith 37
Arabic reference: Book 8, Hadith 326