Ibn Mas’ood reported that the Messenger of Allaah said: “The woman is ‘awrah.”
This is a clear-cut proof that every part of a woman is ‘awrah (i.e. must be covered) in front of male strangers, whether that includes her face, or any otter part of her body. In fact, her face and hands have the most right to be covered since they are the areas that are sought after by men and the places of enticement from women.
Imaam Ahmad Ibn Hanbal understood well the aforementioned hadeeth of Ibn Mas’ood, as he said:
“A woman’s fingernail is ‘awrah (i.e. it must be covered). So when she goes out from her home, she must not reveal any part of herself, not even her khuff (sock), for the chuff describes the foot. What is preferred, in my opinion, is that she put a button on her sleeve where her hand is so that nothing could be exposed from her.”
[Taken from “Four Essays On The Obligation Of Veiling” Ch.4: The Obligation Of Veiling The Hands And Face, by Sh. Zayd Ibn Muhammad Ibn Haadee Al-Madkhalee, Pp. 82-83]