I say, and with God's success: The condition for the validity of a man's prayer is that he covers his private parts, which are from the navel to the knee, and the knee is part of the private parts. If he uncovers anything beyond that during the prayer, it does not invalidate the prayer, but it is disliked as a matter of etiquette. It is also a condition that what he wears does not reveal the color of his skin. And God knows best.