Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.113.

There is no such dispute over what constitutes a woman’s ‘aurah. It is stated that her entire body is ‘aurah and must be covered, except her hands and face. Says Allah in the Qur’an, “And to display of their adornment only that which is apparent (do not expose any adornment or beauty save the hands and face).” It has been authentically related from Ibn ‘Abbas, Ibn ‘Umar and ‘Aishah that the Prophet said, “Allah does not accept the prayer of an adult woman unless she is wearing a headcovering (khimar, hijab).” This is related by “the five,” except for an-Nasa’i, and by Ibn Khuzaimah and al-Hakim. At-Tirmizhi grades it as hassan.

It is related from Umm Salamah that she asked the Prophet, “Can a woman pray in a long shirt (like a night shirt) and headcovering without a loincloth?” He said, “If the shirt is long and flowing and covers the top of her feet.” This is related by Abu Dawud. The scholars say it is sahih in mauqoof form (as a statement of Umm Salamah and not that of the Prophet.)

It is also related that ‘Aishah was asked, “In how many garments is a woman to pray?” She answered, “Ask ‘Ali ibn Abu Talib and then return to me and tell me what he said.” ‘Ali’s answer was, “In a headcover and a long flowing shirt.” This was told to ‘Aishah and she said, “He has told the truth.”

The clothes worn must cover the ‘aurah, even if they are tight enough to highlight those features. If the clothes are so thin that one’s skin colour can be seen, they are not suitable for prayer.