The ruling regarding their prayer is the same regarding the one who prays in shorts without the thobe, because transparent clothes that describe the body are not sufficient and wearing them is equivalent to not wearing them. Based upon this, their prayer is not valid according to the most correct opinion of the scholars, and it is the well known opinion from the madhab of Imaam Ahmad. This is because it is obligatory for men to cover what is between the navel and the knees during the prayer. And this is the lowest form by which compliance to the saying of Allaah, the Mighty and Majestic, occurs, “O Children of Aadam take your adornment while praying…” [al-A’raaf: 31]
So one of two matters is obligatory upon them: either they should wear shorts which cover that which is between the navel and the knees, or they should wear a non-transparent thobe which does not reveal the body. And this action which the questioner has mentioned is a mistake and very dangerous, so it is upon them to repent to Allaah, the Most High, and to be enthusiastic in striving to completely cover themselves in the prayer. We ask Allaah, the Most High, for guidance for us and our Muslim brothers and success to that which He Loves and is Pleased with. Indeed He is the Most Noble, the Most Generous.