Question: Oh Virtuous Shaikh! Many congregants who wear pants [during the prayer], while in the position of prostration or bowing their awrah uncovers from above and they do not know that. So what is the ruling on that?
Answer: “There is no doubt that by wearing pants something will be missed in the prayer. First: complete tranquility while sitting [will be missing]. So like this, you find that those who wear pants sit on their heels [during prayer] along with bending their back a little.
Also, it prevents complete prostration. Sometimes the pants shrink and the top of the awrah uncovers, if what he is wearing on top does not hang down [to cover the awrah].
First: it is necessary not to wear pants because the Qamees is more concealing, spacious, and better for a person.
Second: if he wears pants due to a need like being a policeman and other than that, then let the pants be spacious so that he can do the prayer according to what is necessary. As well, what he wears above it should be apparent [i.e. covering], so that if the pants shrink when he prostrates, that which is above it will be concealing [his awrah]”.
[Liqaa Bab al-Maftooh no. 100]
Faisal Ibn Abdul Qaadir Ibn Hassan