Question: What is the ruling of wearing trousers pants even more so (when) some of whom wear them expose part of their nakedness, and this is at the time of rukoo' and sujood in the salaat?
Answer: If the pants are trousers that conceal what is between the navel and the knee of the man, are loose, not tight, then it is alright to make salaat wearing them. And it is preferable for a shirt to be over them concealing (what is) between the navel and the knees, and extending down past this to the middle of the shin or above the ankle. Because this is more complete in concealing. And salaat with an izaar covering is more preferable than salaat in trousers (without) a shirt covering over them because an izaar is more complete in concealing than trousers.
Source: Fatwa from the book of Da'wah, pg 68