Ruling on swallowing phlegm during prayer

Question

Praise be to Allah.

Phlegm is taahir (pure), because of the report narrated by
al-Bukhaari (409) and Muslim (550), from Abu Hurayrah (may Allah be
pleased with him), according to which the Messenger of Allah (blessings
and peace of Allah be upon him) saw some sputum in the qiblah of the
mosque. He turned to the people and said: “What is the matter with one of
you who stands before his Lord and spits in front of him? Would any one of
you like to have someone stand before him and spit in his face? If one of
you must spit, then let him spit to his left, beneath his foot. If he
cannot do that, then let him do like this,” and al-Qaasim — one of the
narrators of the hadeeth — described how he spat into his garment then
rubbed part of it against another part.

Ibn Qudaamah (may Allah have mercy on him) said:

If it was najis (impure), he would not have told him to rub
it with his garment during prayer or to put it beneath his foot. It makes no
difference whether the phlegm comes from the head or the chest. He also
said:

Phlegm is one of the two types of sputum and is similar to
the other. If it was najis, the mouth would be made impure by it, … and
there is no report from the Sahaabah (may Allah be pleased with them) to
that effect, even though it is a common problem. End quote.

Al-Mughni (1/415)

Swallowing phlegm during prayer does not come under the same
ruling as food and drink, because it is not food or drink.

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked:

The fuqaha’ said that sputum invalidates the fast of a
fasting person. Is it possible to draw an analogy with prayer, i.e., does
sputum invalidate the prayer?

He replied:

Firstly:

The fuqaha’ are not unanimously agreed on this. Rather the
madhhab of Imam Ahmad has two opinions as to whether it breaks the fast or
not.

Secondly:

What is meant by the kind of sputum that breaks the fast is
that which reaches the mouth. As for that which appears in the throat and
goes down into the chest, that does not break the fast. I do not think the
sputum reaches the mouth of anyone and is then swallowed, because it is
off-putting. But whatever the case, most of the Hanbali fuqaha’ say that if
sputum reaches the mouth and is then swallowed, it invalidates the fast.

With regard to the analogy of that, that if that happens
during prayer, it invalidates the prayer, if we say that this is like food.
But I have not come across anyone discussing this issue with regard to
prayer. Moreover the idea that it breaks the fast if it reaches the mouth
then is swallowed is subject to further discussion, because this is not
regarded as eating or drinking. It did not enter his body from outside,
rather it is still inside him, and the mouth is regarded as part of the
outside, not part of the inside. End quote.

Liqa’ al-Baab al-Maftooh,
116/17

As phlegm is taahir and is not food or drink, and does not
come under the same rulings as food and drink, if a person swallows whilst
praying, his prayer remains valid, especially if he could not help it and
was not able to spit it out into a handkerchief and the like. Swallowing
phlegm is usually of-putting, and what is prescribed is to spit it out into
a handkerchief and the like, not swallow it.

And Allah knows best.