On Wearing Trousers in Prayer
Does wearing trousers in Prayer cause one's ʿawrah (private area) to be exposed?
This is a more complete form of covering the ʿawrah. Likewise, prayer in an ʿizār (a loose lower garment) is more preferable than prayer in a pair of trousers without a long short - since the ʿizār is far better in covering the ʿawrah than a pair of trousers.
Shaykh ʿAbd al-ʿAzīz ibn ʿAbdullāh ibn Bāz
Sometimes whilst praying - especially during rukūʿ (bowing) and sujūd (prostration) - part of the ʿawrah (private parts) of person becomes exposed due to his wearing trousers. So what is the ruling of wearing trousers while praying?
"If a man’s trousers cover what is between his knees and his navel and are loose-fitting, wide and baggy, then it fulfills the condition for the correctness of the Prayer. However, what is more preferable is to where a qamīs (long shirt) - which reaches half way down to the shins, or just above the ankle-bone - over the trousers. This is a more complete form of covering the awrah. Likewise, prayer in an izār (a loose lower garment) is more preferable than prayer in a pair of trousers without a long short - since the izār is far better in covering the awrah than a pair of trousers."1
 al-Fatāwá (1/68-69)