Shaykh Al-Islaam Ibn Taymiyyah said:
“The khimaar is that which covers the head, the face and the neck. And the jilbaab is that which is made to fall from the top of the head downwards, such that oohing from her dress is apparent except forger two eyes.” [As is reported on him by Al-Albaanee in his book “Hijaab” p.71]
Imaam Ibn Khuzaymah said in his saheeh:
“The khimaar: that which a woman covers her face with.”
And this is what the female companions understood regarding khimaar when the ayah obligating it was revealed.
“May Allaah have mercy on the first emigrant women (muhaajiraat). When Allaah revealed “And (tell them) to draw their khimaars over their bosoms (juyoob)”, they tore off the bottom parts of their dresses and used them as khimaars (and covered their faces with them).” [Bukhaaree]
Ibn Hajr said:
“Her saying ‘used them as khimaars’ means we covered our faces with them.” [Taken from the book ‘Awdatul Hijaab 3/287-288]
Imaam Muhammad Al-Ameen Ash-Shinqeetee said (when commenting on the above hadeeth of ‘Aa’ishah):
“This authentic hadeeth is clear proof that the female companions mentioned in it understood that the meaning of Allaah’s saying, “And (tell them) to draw their khimaars all over their juyoob (bosoms)” required the veiling of their faces. That is why they tore off part of their dresses and “used them as khimaars” meaning they covered their faces with them, obeying Allaah’s command in (24:31), which mandates the veiling of their faces.
So a woman’s wearing of the hijaab in front of men and the veiling of her face before them is established in the authentic sunnah, which provides an explanation for the Quraan… It is well known that they could not have gotten their understanding that Allaah’s saying “And (tell them) to draw their khimaars all over their juyoob (bosoms)” meant to veil the face except from the Prophet (saw), since he was present at that time. They would ask him about things that they found difficult with in their religion… So it is not possible that they could come up with an understanding of this ayah by themselves.” [Adwaa Al-Bayaan, 6/595]