Can he pray in the clothes he slept in?

Question

Praise be to Allah.

Firstly:

The basic principle with regard to clothes is that they are
taahir (pure), unless any najaasah (impurity) has gotten onto them, in which
case they must be purified.

Merely sleeping in clothes does not make them najis (impure);
rather they remain pure until it becomes certain that any impurity has
gotten onto them.

Shaykh Ibn ‘Uthaymeen said: The basic principle with regard
to a person wearing clothes is that he (and they) are pure so long as he is
not certain that any impurity has gotten onto his body or clothes. This
basic principle is supported by the words of the Prophet (blessings and
peace of Allah be upon him) when a man complained to him that he felt
something during his prayer – referring to breaking wind. He said: “Do not
stop (praying) unless you hear a sound or notice a smell.” If the individual
is not certain about that, then the basic principle is that he remains pure.
He may think it most likely that his clothes have become contaminated with
impurity, but so long as he is not certain about it, the basic principle is
that he remains pure. End quote.

Majmoo‘ Fataawa Ibn ‘Uthaymeen,
11/107

For more information please see the answer to question no.
12720

Secondly:

It is permissible to pray in clothes in which one has slept,
so long as they are pure and cover the ‘awrah, although it is better and
more perfect to wear beautiful clothes that look good for prayer. That is in
obedience to the command of Allah, may He be exalted, in the verse
(interpretation of the meaning): “O Children of
Adam! Take your adornment (by wearing your clean clothes), while praying”
[al-A‘raaf 7:31].

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked
about the ruling on praying in nightclothes and attending gatherings wearing
them. He said: There is nothing wrong with praying in nightclothes if they
are pure, whether one comes to the mosque wearing them or not, unless those
clothes will attract attention and criticism, and people will talk about him
in their gatherings because of that. People should avoid anything that may
be a cause of people backbiting about them. End quote.

Majmoo‘ Fataawa Ibn ‘Uthaymeen,
12/362

And Allah knows best.