Modesty

And tell the believing women to cast down their glances and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their head covers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women." -Quran 24:31.

“O Prophet! Say to your wives, your daughters, and the women of the believers that: they should let down upon themselves their jalabib.” -Quran 33:59. Jalabib is an Arabic word meaning "loose outer garment".

In some Muslim societies, women wear the Niqab  a veil that covers the whole face except the eyes, or the full Burqa  a full-body covering garment that occasionally does cover the eyes. Wearing these garments is common in some, but not all, countries with a predominantly Muslim population.