Is there any ruling that the hands have to be uncovered while praying? Some say the hands should not be covered or hidden under a long scarf.
No Scholar ever said that you should uncover your hands in prayers, i.e. that you are not allowed to cover them.
Actually there is a difference of opinion as whether it is obligatory to cover them or not. Shaikh Ibn Uthaymeen says that the strongest opinion is that you do not have to cover them.