Question
A man is praying in his underwear (a tank top), and every time he prostrates, his lower back is exposed. What is the ruling on his prayer?
Answer
I say, and with God's help: Anything that is below what is equivalent to the navel is considered part of the 'awrah, and it is forbidden to uncover it. If the uncovered area exceeds a quarter of a limb, the prayer is invalidated. If the uncovered area is more than a quarter of one of the buttocks, the prayer is invalidated. And God knows best.