What is the ruling on a woman praying without underwear, while being fully covered by a loose garment specifically for prayer?

A woman must cover her entire body during prayer, except for her face and hands. It is permissible for her to pray in a single garment as long as it covers her entire body. Sheikh Ibn Uthaymeen, may Allah have mercy on him, was asked about the prayer garment for women, and he replied that it is permissible for the garment in which a woman prays to be a single piece if it covers what must be covered, and her prayer is valid.