{ Oh Prophet, say to your wives and your daughters and the women of the believers to pull their outer garments over themselves; this is more suitable, that they may be recognized and not harmed } Al-ahzab 59

‘Ali ibn Abi Talha related from Ibn ‘Abbas that Allah ordered the women of the believers, whenever they went out from their houses for a need, to cover their faces from the top of their heads with their outer garment [jilbab], showing one eye only. Muhammad ibn Sirin said: I asked ‘Ubaida Al-Salmani about the word of Allah Most High { Pull their outer garments over themselves } , and he covered his face and his head and showed just his left eye.
… Regarding the word of the Most High { This is more suitable, that they may be recognized and not harmed } : that is, whenever they did this they would be recognized as being free women, not slave girls and not whores.