Maliks Muwatta Book 8, Hadith Number 38.

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

Yahya related to me from Malik from a reliable source from Bukayr ibn Abdullah ibn al-Ashajj from Busr ibn Said that when Ubaydullah ibn al-Aswad al-Khawlani was in the room of Maimuna, the wife of the Prophet, may Allah bless him and grant him peace, she used to pray in a shift and head-covering, without a waist-wrapper.