I am a person who sweats, especially on hot days. When I sweat, my whole body sweats, including the anus, and the sweat that is in the anus gets onto my underwear. When my underwear gets stuck to the damp anus, this causes a problem. Sometimes the sweat has some colour of the najaasah (impurity) – i.e., brown – but it is very faint, and can only be seen under the light. There is a smell too, but sometimes there is no smell, only sweat. And sometimes only the colour is left, even after washing the clothes. I am confused about this matter.

Can I pray if I sweat, or not? Or should I check my clothes – although that is not really proper – or what? Please note that the anus may sweat because of minor things such as walking, carrying things or standing in the sun.

Does this invalidate the prayer? What is the ruling on the prayers that I have offered? Sometimes the color appears without any smell. What do you advise?

Please note that I feel very embarrassed, and I interrupt my prayer in order to check my clothes, because I realized that I am sweating. Sometimes I am compelled to change my clothes more than once a day.

Praise
be to Allah.

Firstly: 

What
gets onto underwear of the traces and smell of najaasah (impurity) due to
sweating is usually very little, and can be overlooked, as is the view of
the Hanafis and many of the scholars. 

Al-Kaasaani
said: Because a little najaasah cannot be completely avoided. Flies may land
on something impure, then land on the clothes of the worshipper, and there
will inevitably be some impurity on their wings and feet; if that were not
overlooked, it would cause hardship for people.

End
quote from Badaa’i‘ as-Sanaa’i‘, 1/79 

Shaykh al-Islam Ibn Taymiyah said: A little najaasah may be overlooked, even
mouse droppings and the like in food and so on. This is one view in the
madhhab of Ahmad. If there is certainty that mud in the street is najis
(impure), a little may be overlooked because it is too difficult to avoid it
completely.

End
quote from al-Fataawa al-Kubra, 5/313 

Shaykh Ibn ‘Uthaymeen said: The correct view is that of Abu Haneefah and
Shaykh al-Islam… Included in the small amounts of impurities that may be
overlooked because it is too difficult to avoid it completely is a small
trace of urine for one who suffers incontinence, but has done his best to
protect himself from it as much as he can.

End
quote from ash-Sharh al-Mumti‘, 1/447 

Secondly: 

Something that may make the matter more flexible with regard to what you
mentioned is the fact that Islam allows a man to clean himself (after
relieving himself) with stones or pebbles. It is well known that stones do
not purify the place completely; rather there will inevitably still be some
small traces of impurity, but this is something that may be overlooked. 

Ibn
Qudaamah said: Traces of impurity from urination and defecation may be
overlooked because of where they are coming from in three cases, one of
which is istinja’ (cleaning oneself after relieving oneself). Traces left
after cleaning oneself with stones in the prescribed manner and number may
be overlooked, and there is no difference of scholarly opinion concerning
that, as far as we know.

End
quote from al-Mughni, 2/486 

Al-Qarraafi
said: If a person sweats in his garment after cleaning himself with stones,
it is overlooked, because this is a problem that everyone has… And because
the Sahaabah (may Allah be pleased with them) used to clean themselves with
stones and they used to sweat.

End
quote from adh-Dhakheerah, 1/211 

It
says in Sharh Mukhtasar Khaleel by al-Kharashi (1/148): If a person sweats
in that place, and it gets onto the garment, it does not matter. 

Ibn
Qudaamah said: Because the Sahaabah (may Allah be pleased with them) mostly
used stones to clean themselves… and their land is hot, so it seems that
they were not safe from sweating, but there is no report from them to
suggest that they tried to avoid that or protect themselves from it.

End
quote from al-Mughni, 1/119 

Ibn
al-Qayyim said: Most of the Sahaabah did not clean themselves with water
(after relieving themselves); rather they used stones in the summer and in
the winter; and usually a person sweats in his izaar or lower garment, but
the Prophet (blessings and peace of Allah be upon him) did not instruct them
to wash it although he was aware of the situation, and they did not do that,
even though they were the best of generations and the ones who feared Allah
most.

End
quote from Badaa’i‘ al-Fawaa’id, 4/1490 

Shihaab ad-Deen ar-Ramli ash-Shaafa‘i was asked about a person who cleaned
himself with stones, then the tip of his penis touched a wet spot on his
body whilst he was praying – did that invalidate his prayer, and did he have
to clean himself with water and wash the area it got onto?

 He
replied: It does not invalidate his prayer, and he does not have to clean
himself with water or wash whatever got onto the area that was cleaned with
stones, because they (the scholars) said: Traces left after cleaning oneself
with stones are overlooked, even if he sweats in the place and some slight
contamination affected some other part.

End
quote from Fataawa ar-Ramli, 1/33 

Shaykh Muhammad Mukhtaar ash-Shinqeeti said: 

Once
it is established that a small amount of impurity in the anus or on the tip
of the penis may be overlooked if a person cleans himself with stones then
sweats, then that sweat will inevitably reach that place. If a place near it
sweats, and this sweat reaches the garment or pants near that spot, this is
overlooked, because if we were to rule that it is impure, this would cause
hardship for the people the extent of which no one would know but Allah, may
He be glorified and exalted. 

End
quote from Sharh Zaad al-Mustaqni‘, 23/4) 

See
also al-Mughni by Ibn Qudaamah, 1/219 

To
sum up: this slight trace that results from sweat does not matter, and your
prayer is valid. All you need to do is make sure that you clean yourself
properly when you clean yourself after relieving yourself, then do not worry
about it any more than you should after that, so that you will not fall prey
to waswaas. 

And
Allah knows best.