Praise be to Allah.
We advise you to adhere to complete hijaab in front of non-mahram men so that your fast will be accepted and its reward multiplied, and so that the same will apply to your prayer and the rest of your good deeds. If a Muslim woman fasts but does not wear hijaab, her fast is still valid, but she is sinning by neglecting hijaab. Being uncovered does not affect the validity of one’s fast, but the one who displays her beauty and adornments (tabarruj) is threatened with punishment from Allaah for going against His commandment. We advise you to obey the commands of Allaah (interpretation of the meanings):
“…to draw their cloaks (veils) all over their bodies…”
“…and not to show off their adornment…”
“…and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms)…”