What is the ruling on a woman who prays without wearing a head covering?

It is obligatory upon the Muslim woman to stay away from tabarruj (wanton display) and sufur (unveiling), and everything that displeases Allah. A woman removing her Shar'i (Islamically prescribed) hijab is among the reprehensible acts. If she prays in clothing that covers her except for her face and hands, then goes out without hijab, she is disobedient, but her prayer is valid. However, it may not yield its full fruits because prayer prevents fahsha' (indecency) and munkar (reprehensible acts). Ibn Mas'ud said: "Whoever's prayer does not prevent him from fahsha' and munkar gains nothing from Allah but distance." As for she who does not wear hijab neither in prayer nor outside of it, the matter is more severe due to her failure to fulfill the condition of covering her awrah (intimate parts).