Praise be to Allah.
It is not correct to cite the fact that the woman uncovers her face when praying as proof that the face is not ‘awrah. To explain further:
There is no text in either the Holy Qur’an or the Prophet’s Sunnah to indicate that the worshipper is enjoined to cover his ‘awrah and uncover everything else. Therefore it is not valid to say that that which the worshipper is enjoined to cover (when praying) is ‘awrah and that what he is permitted to leave uncovered is not ‘awrah.
Rather the command regarding that in the Qur’an mentions adorning and beautifying oneself for prayer. Allah, may He be exalted, says (interpretation of the meaning):
“O children of Adam, take your adornment at every masjid [[YA at every time and place of prayer]]”
Taking or wearing adornment is different from covering the ‘awrah. Therefore the worshipper is enjoined to cover that which is not ‘awrah. The Prophet (blessings and peace of Allah be upon him) forbade men to pray with their shoulders uncovered, even though a man’s shoulder is not ‘awrah, according to scholarly consensus.
A woman’s head and hair are not ‘awrah in the presence of her husband and mahrams such as her father and brother, yet despite that it is not permissible for her to pray in front of her husband or mahrams with her hair uncovered. In fact, she is not allowed to pray with her head uncovered even if she is on her own where no one can see her.
From this it is known that citing the fact that the woman prays with her face uncovered as evidence that the face is not ‘awrah is not correct, because prayer has its own rulings, which are different from the rulings on covering the ‘awrah outside of prayer.
This was explained by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), who said:
If a woman prays by herself, she is enjoined to cover her head, whereas outside of prayer it is permissible for her to uncover her head in her house. So taking or wearing adornment when praying is following the command of Allah. Hence it is not permissible for anyone to circumambulate the Ka‘bah naked, even if he is alone at night, or to pray naked even if he is alone. Thus it is known that wearing adornment when praying is not for the purpose of covering oneself in front of other people, because this is one thing, and that (covering oneself in front of other people) is another thing. Therefore, the worshipper may be enjoined to cover when praying that which it is permissible to show when not praying, and when praying, a man may show that which he covers in front of other men. An example of the former is the shoulders (of a man). The Prophet (blessings and peace of Allah be upon him) forbade a man to pray in a single garment with nothing of it on his shoulders. This is something that is enjoined specifically with regard to prayer, because it is permissible for a man to uncover his shoulders in the presence of other men outside of prayer. Likewise, a woman should cover her head when praying, as the Prophet (blessings and peace of Allah be upon him) said: “Allah does not accept the prayer of a woman who menstruates [i.e., a woman of child-bearing age] except with a headcover.” But the woman does not cover her head in the presence of her husband or of her mahrams. And it is permissible for her to show her hidden adornment to these men, but when she is praying, it is not permissible for her to uncover her head in the presence of these men or any others.
The face, hands and feet are the opposite of that. The woman cannot show them to non-mahrams according to the sounder of the two scholarly views. As for covering those parts when praying, that is not obligatory, according to the consensus of the Muslims; rather it is permissible for her to show her face and hands when praying according to the majority of scholars, such as Abu Haneefah, ash-Shaafa‘i and others; this is also one of the two views narrated from Ahmad. It is also permissible for her to show her feet [when praying] according to Abu Haneefah.
To sum up, it is proven on the basis of the religious texts and scholarly consensus that when she prays, a woman does not have to wear the jilbab that covers her completely, if she is at home; rather that is only required if she goes out. But in this instance, she is praying in her house, and even if she shows her face, hands and feet, that is fine, as they used to walk about in the beginning, before the command was revealed to “bring down over themselves [part] of their outer garments” [al-Ahzaab 33:59]. So what is regarded as ‘awrah in prayer has nothing to do with what is regarded as ‘awrah that is to be covered in front of non-mahrams.
In the context of prayer, the fuqaha’ referred to “covering the ‘awrah”, but this is not the wording that was used by the Messenger (blessings and peace of Allah be upon him), and in the Qur’an and hadith, what the worshipper is required to cover is not referred to as ‘awrah. Rather Allah, may He be exalted, says (interpretation of the meaning): “O children of Adam, take your adornment at every time and place of prayer” [al-A‘raaf 7:31]. And the Prophet (blessings and peace of Allah be upon him) forbade people to circumambulate the Ka‘bah naked, so it is more appropriate that no one should be allowed to pray naked. The Prophet (blessings and peace of Allah be upon him) was asked about praying in a single garment, and he said: “Does each of you have two garments?”
And he said concerning (praying in) a single garment: “If it is large enough, then wrap it around the body (covering the shoulders), and if it is too short, then tie it around the waist only.” And he forbade any man to pray in a single garment with no part of it over his shoulders. This indicates that it is enjoined to cover the ‘awrah when praying, including the thighs and other parts, even if we assume that it is permissible for a man to see those parts [of another man]. So if we assume the soundness of the view – which is one of two views narrated from Ahmad – that the ‘awrah is the private parts, front and back, and that the thigh is not ‘awrah, this has to do with the permissibility of another man looking at them, and has nothing to do with prayer or tawaaf. It is not permissible for a man to pray with his thighs uncovered, regardless of whether it is said that they are ‘awrah or not, and it is not permissible for him to do tawaaf naked. Rather he must pray in a single garment when he has no alternative; if it is too short, he should wrap it around his waist, and if it is large enough, then he should wrap it around his body (covering the shoulders). By the same token, if he is praying alone in a room, he must cover that, according to scholarly consensus. As for a man praying with his thighs uncovered when he is able to wrap something around his waist and cover them, that is not permissible and there should not be any difference of opinion concerning that. Whoever suggested that on the basis of the two reports concerning the ‘awrah – as one group did – got it wrong. Neither Ahmad nor anyone else suggested that a worshipper may pray in this manner. How could that be the case when Ahmad enjoined the worshipper to cover his shoulders; how could he permit him to uncover his thighs [when praying]?! There is a difference of scholarly opinion regarding the obligation of covering the ‘awrah if a man is on his own, but there is no difference of opinion that when he is praying he must be covered up. It is not permissible to pray naked when one is able to find a garment, according to scholarly consensus. End quote.
Majmoo‘ al-Fataawa (22/113-117).
Ibn al-Qayyim (may Allah have mercy on him) said:
The ‘awrah is of two types: the ‘awrah before other people and the ‘awrah when praying. A woman may pray with her face and hands uncovered, but she does not have the right to go out like that in the marketplaces and places where people gather. End quote.
I‘laam al-Muwaqqi‘een (2/80)
And Allah knows best.