The requirement for the garment covering the 'awrah.
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
It is obligatory that the covering conceals the color of the skin. If the fabric is so thin that the whiteness or redness of the skin is visible through it, the prayer is invalid because proper covering is not achieved. If the garment conceals the skin color but outlines the body shape (describes the form), the prayer is valid because avoiding the description of the form is impossible even with thick material.