Shaykh Saalih al-Fawzaan (may Allaah preserve him) said:

Shaykh Saalih al-Fawzaan (may Allaah preserve him) said:

The correct view as indicated by the evidence is that the woman’s face is ‘awrah which must be covered. It is the most tempting part of her body, because what people look at most is the face, so the face is the greatest ‘awrah of a woman. This is in addition to the shar’i evidence which states that it is obligatory to cover the face.
For example, Allaah says (interpretation of the meaning):

“And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, headcover, apron), and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms)…”

[al-Noor 24:31]

Drawing the veil all over the juyoob implies covering the face.

When Ibn ‘Abbaas (may Allaah be pleased with him) was asked about the aayah (interpretation of the meaning):

“O Prophet! Tell your wives and your daughters and the women of the believers to draw their cloaks (veils) all over their bodies”

[al-Ahzaab 33:59] –

he covered his face, leaving only one eye showing. This indicates that what was meant by the aayah was covering the face. This was the interpretation of Ibn ‘Abbaas (may Allaah be pleased with him) of this aayah, as narrated from him by ‘Ubaydah al-Salmaani when he asked him about it.
(Collected by Ibn Abee Shaybah in al-Musannaf, Vol. 3, p. 540 & 541, hadith no. 16997 & 17012 and al-Bayhaqi in Sunan al- Kubraa. Al-Albaanee ruled in Jilbaab al-Mar'ah al-Muslimah, pp 59-60, that the isnaad of this statement is saheeh.)

http://tafsir.com/default.asp?sid=33&tid=42193

In the Sunnah there are many ahaadeeth, such as: the Prophet (peace and blessings of Allaah be upon him) said: “The woman in ihraam is forbidden to veil her face (wear niqaab) or to wear the burqa’.” This indicates that when women were not in ihraam, women used to cover their faces.
This does not mean that if a woman takes off her niqaab or burqa’ in the state of ihraam that she should leave her face uncovered in the presence of non-mahram men. Rather she is obliged to cover it with something other than the niqaab or burqa’, on the evidence of the hadeeth of ‘Aa’ishah (may Allaah be pleased with her) who said: “We were with the Prophet (peace and blessings of Allaah be upon him) in ihraam, and when men passed by us, we would lower the khimaar on our heads over our faces, and when they moved on we would lift it again.”

Women in ihraam and otherwise are obliged to cover their faces in front of non-mahram men, because the face is the center of beauty and it is the place that men look at… and Allaah knows best.

[Fataawa al-Mar’ah al-Muslimah, 1/396, 397]

Hadith
"One night Sauda bint Zam'a the wife of the Prophet went out at 'Isha' time and she was a tall lady. 'Umar addressed her and said, "I have recognized you, O Sauda." He said so, as he desired eagerly that the verses of Al-Hijab (the observing of veils by the Muslim women) may be revealed. So Allah revealed the verses of "Al-Hijab" (A complete body cover excluding the eyes). "

Sahih Bukhari 1:4:148
" Bodily characteristics of a lady and saying: As the comes in front four folds appear on her front side and as she turns her back eight folds appear on the back side. Thereupon Allah's Apostle (may peace be upon him) said: I me that he knows these things; do not, therefore. allow him to cater. She (" A'isha) said: Then they began to observe veil from him."

Sahih Muslim 26:5416