All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The ‘Awrah of women during prayer is her entire body except for the face and the hands (i.e. palms of the hand from the wrist bone to the finger-tips). Therefore, revealing a woman's wrist during prayer is no different from revealing the palms of the hand in terms of permissibility.