Muwatta Malik 323

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ مُحَمَّدِ بْنِ زَيْدِ بْنِ قُنْفُذٍ، عَنْ أُمِّهِ، أَنَّهَا سَأَلَتْ أُمَّ سَلَمَةَ زَوْجَ النَّبِيِّ صلى الله عليه وسلم مَاذَا تُصَلِّي فِيهِ الْمَرْأَةُ مِنَ الثِّيَابِ فَقَالَتْ تُصَلِّي فِي الْخِمَارِ وَالدِّرْعِ السَّابِغِ إِذَا غَيَّبَ ظُهُورَ قَدَمَيْهَا ‏.‏

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

Reference

Muwatta Malik 323

In-book

Book 8, Hadith 37