What are the limits of the ‘awrah during prayer?

Firstly: 

‘Awrah in Arabic means a weak point along a border and the
like. 

In Misbaah al-Muneer it says: Everything that a person
covers out of dignity or modesty is ‘awrah. 

According to the fuqaha’, everything that it is haraam for a
man or woman to uncover is ‘awrah. 

Covering the ‘awrah, in the terminology of the fuqaha’, means
covering that of which exposure is shameful and causes shame, whether the
individual is male or female or hermaphrodite. 

See: al-Mawsoo‘ah al-Fiqhiyyah, 24/173 

Secondly: 

Covering the ‘awrah is one of the conditions of prayer being
valid because Allah, may He be exalted, says: (interpretation of the
meaning):

“Take your adornment (by wearing your clean clothes) while
praying”

[al-A‘raaf 7:31]. 

Ibn ‘Abbaas (may Allah be pleased with him) said: What is
meant by adornment in this verse is wearing clothes whilst praying. This was
narrated by al-Tabari in his Tafseer, 12/391 

And the Prophet (sa) said: “Allah does not accept the prayer
of a woman of childbearing age (lit. a woman who menstruates) except with a
head covering.”

Narrated by Abu Dawood, 641; narrated and classed as saheeh
by al-Tirmidhi, 377; classed as saheeh by al-Albaani. 

It says in al-Mughni (1/336):

Concealing the ‘awrah from view with something that does not
show the skin is obligatory and is a condition of prayer being valid. This
is the view of al-Shaafa‘i and ashaab al-ra’y. End quote. 

Ibn Hajar said: 

The majority are of the view that covering the ‘awrah is one
of the conditions of prayer.

End quote. Fath al-Baari, 1/466 

Thirdly: 

What is required of the worshipper is to cover his ‘awrah
whilst praying, according to the consensus of the Muslims. The ‘awrah of a
man is the area between the navel and the knees, according to the majority
of scholars. 

See: al-Mughni, 3/7; al-Istidhkaar, 2/197;
Fataawa Islamiyyah, 1/427 

With regard to women, the hair and the entire body are ‘awrah
and she has to cover them, apart from the face and hands. If she does that,
then her prayer is proper, according to scholarly consensus. 

See: al-Iqnaa‘ fi Masaa’il al-Ijmaa‘, by Ibn Qattaan,
1/121-123; al-Sharh al-Mumti‘, 2/160 ff.

 Fourthly: 

If a person starts to pray with his ‘awrah covered, then
during the prayer he is not sure whether some of it has become uncovered,
then he should ignore that doubt and complete his prayer, because the basic
principle is that the ‘awrah is still covered, and developing doubt about
what you were certain about should not be given any attention. 

Al-Bukhaari (137) and Muslim (361) narrated from ‘Abbaas ibn
Tameem that his paternal uncle said: A complaint was made to the Prophet (S)
about when one thinks that something has happened whilst he praying. He
said: “Do not stop until you hear a sound or notice a smell.” 

Al-Nawawi said: 

This hadeeth is one of the basic principles of Islam and an
important foundation of fiqh, namely that things are deemed to still be as
they originally were unless there is certain proof of the contrary, and
passing doubts do not affect that. End quote. 

What the worshipper has to do is to prepare for his prayer
before he starts to pray. So he should put on clothes that he is certain
will cover his ‘awrah and leave clothes that he is afraid will let something
of his ‘awrah show whilst he is praying, such as short T-shirts and other
kinds of clothing which may rise up and show the lower back, lest some part
of his ‘awrah be uncovered when he bows or prostrates. 

See also the answer to questions no.
3075 and
107701.