Maliks Muwatta Book 8, Hadith Number 39.
Section : Permission for a Woman to Pray in a Shift and Head-Covering.
Yahya related to me from Malik from Hisham ibn Urwa from his father that a woman asked him for a decision, saying, “Waist-wrappers are painful to me. Can I pray in a shift and head-covering?” He replied, “Yes, if the shift is long.”