Allah states in the Holy Quran: 

"And tell the believing women to lower their gaze (from looking at forbidden things) and protect their private parts (from illegal sexual acts, etc.) and not to show off their adornments except only that which is apparent (like palms of hands, etc.), and to draw their veils all over Juyubihinna (i.e. their bodies, necks and bosoms, etc.) and not to reveal their adornment except to their husbands, their fathers, their husband’s fathers, their son’s, their husband’s sons, their brothers or their brother’s sons, or their sisters sons or their (Muslim) women (i.e. their sisters in Islam), or the (female) slaves whom their right hands possess, or old male servants who lack vigour, or small children who have no sense of the shame of sex. And let them not stamp their feet so as to reveal what they hide of their adornment. And beg Allah to forgive you all O believers, that you may be successful.