Maliks Muwatta Book 8, Hadith Number 37.

Section : Permission for a Woman to Pray in a Shift and Head-Covering.

Yahya related to me from Malik from Muhammad ibn Zayd ibn Qunfudh that his mother asked Umm Salama, the wife of the Prophet, may Allah bless him and grant him peace, “What clothes can a woman wear in prayer?” She said, “She can pray in a shift that reaches down and covers the top of her feet.”