Bulugh al-Maram

205 Benefits and Issues:
➊ A mosque is constructed for the worship and servitude of Allah, and it is commanded to keep it free from both outward and inward filth and impurity.
➋ Spitting in the mosque is against the etiquette of the mosque, propriety, and cleanliness; moreover, it is also offensive to sound taste. Therefore, spitting is a sin, and its expiation is to bury it so that no trace of it remains.
➌ This ruling pertains to the time when the floor was of earth; now, if someone spits on a finished (paved) floor, it is necessary to clean it with water or cloth, etc.

Hadith Commentary:
1.
Spitting in the mosque is against its sanctity and respect, but if, due to some necessity, it becomes unavoidable, then its remedy and expiation is that it should be buried.
This applies in the case where the floor is covered with sand or earth.
And if the mosque floor is solid (paved), then it should be picked up and thrown outside.
In fact, Imam Bukhari rahimahullah has pointed towards a difference of opinion.
According to Allamah Nawawi rahimahullah, spitting during prayer and in the mosque is, in any case, a sin, whether one intends to bury it or not.
If he buries it, then the sin will be compensated, whereas Qadi Iyad rahimahullah says that if, at the time of spitting, he intended to remove it, then there is no sin; however, if he does not remove it, then he will be sinful.
The basis of this difference is that there is a contradiction in the generality of the ahadith narrated from the Messenger of Allah sallallahu alayhi wa sallam in this regard.
One hadith states that spitting in the mosque is a sin, and another statement is that in case of necessity, spitting to the left side or under the left foot is permissible.
Imam Nawawi rahimahullah, keeping the first statement general, has specified the second by saying that the permission to spit to the left side or under the foot is in the case when one is not in the mosque, whereas Qadi Iyad rahimahullah, keeping the second statement general, has specified the first by saying that if, after spitting, there is no intention to bury it in the ground, then spitting in the mosque is a sin.
Hafiz Ibn Hajar rahimahullah has stated that many of the earlier scholars have supported Qadi Iyad, and then he has referenced some narrations from which Qadi Iyad’s position is considered stronger.
(Fath al-Bari: 1/662-663)
In any case, according to the hadith, spitting in the mosque is a sin, and if someone commits this sin, its expiation is to bury it.
Thus, burying it is akin to repentance after the sin.
Imam Bukhari rahimahullah has established from these ahadith that during prayer or in the mosque, if there is a necessity to spit, it is essential to observe the etiquette of the mosque and consideration for the worshippers.
2.
In the early days of Islam, there were few mosques, and their courtyards had sand or earth, in which it was possible to spit and then conceal it in the sand or earth.
Whereas nowadays, mosques are solidly built, and their floors are also solid, covered with fine mats or carpets; in such circumstances, it is appropriate to spit into a handkerchief, etc.
Therefore, in such conditions, making use of the aforementioned ahadith, spitting on the walls or floors of mosques is a grave sin and a desecration of the mosques.
The Messenger of Allah sallallahu alayhi wa sallam expressed severe displeasure at those who violate the sanctity of mosques in this manner.
Therefore, due to the change in circumstances, now spitting to the left side or under the foot is absolutely inappropriate.
Now, only two options should be practiced:
Either swallow the spit or phlegm, or use a handkerchief or tissue paper for it.

Hadith Commentary:
Benefits and Issues:
From this hadith, it is understood that a person should not spit in the mosque. If, due to necessity and compulsion, he does spit, then according to another hadith, he should spit beneath his left foot and bury it.
Therefore, there is no contradiction between the two hadiths, and there is some difference of opinion on this issue between Qadi Iyad rahimahullah and Imam Nawawi rahimahullah. According to Imam Nawawi rahimahullah, one should not spit in the mosque; if there is a need, he should spit into his own garment.
If he spits in the mosque, he will be sinful. Qadi Iyad's view is that if he spits in the mosque and buries it, then there will be no sin; if he does not bury it, then he will be sinful. Nowadays, it is not possible to bury (spittle) in the mosques,
therefore, in cases of necessity and compulsion, one should use a spittoon or a piece of cloth.

1 ᵎ:
That is, to cover (the grave) by putting soil or the like over it.

475. Commentary:
It is evident that this ruling pertains to those mosques whose floors are unpaved (earthen). If this impurity occurs on a paved floor, then it is necessary that it be thoroughly wiped or washed.

724. Commentary: Spitting is a cause of impurity; therefore, spitting in the mosque is prohibited. If the mosque has an earthen floor, it can be buried in the soil, and if the floor is paved, it should be cleaned with a cloth or something similar. If, during prayer, one is unable to restrain the spit, then one should spit into one's garment and rub it in so that even the garment does not feel dirty, or if there is tissue paper, one should spit into it, and this is better.