Prayer: Clothing Covering Private Parts Only While Standing
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 780
If some clothing covers one’s private parts while he is standing, but it is possible that it would not cover them in other positions – for example, in the position of rukūʿ and sujūd (prostrating) – then, in the event that when his private parts are visible, he covers them by some means, his prayer is valid. However, the recommended precaution is that he should not perform prayers with such clothing.