Sometimes whilst praying – especially during rukoo’ (bowing) and sujood (prostration) – part of the awrah(private parts) of a person becomes exposed due to his wearing trousers. So what is the ruling concerning the wearing of trousers whilst praying?
“If the man’s trousers cover what is between his navel and his knees 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 wear a qamees (long shirt) – which reaches half-way down to the shins, or just above the anklebone – over the trousers. This is a more complete form of covering the awrah.
Likewise, Prayer in an izaar (a loose lower garment) is more preferable than Prayer in a pair of trousers without a long shirt – since the izaar far better covers the awrah than a pair of trousers.”