Chapter: The prohibition of spitting in the mosque, the duty to remove it when found, and the command to keep the mosque free of impurities.

1693 – Anas (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) said, “Spitting in the mosque is a sin, and its expiation is to bury it.” Agreed upon. What is meant by “burying it” is that if the mosque floor is soil, sand, or the like, one should cover the spit with earth. Abū al-Maḥāsin al-Ruʾyānī said in his book al-Baḥr, “Some have explained ‘burying it’ to mean taking it out of the mosque. If, however, the mosque is paved or plastered and someone rubs the spit into the floor with his shoe or anything else—as many ignorant people do—this is not burial; rather, it adds to the sin and increases the filth in the mosque. Whoever does so must then wipe it away with his garment, his hand, or something else, or wash it.”

1694 – ʿĀʾishah (may Allah be pleased with her) reported that the Messenger of Allah (peace and blessings be upon him) saw mucus, spittle, or phlegm on the qiblah wall and scraped it off. Agreed upon.

1695 – Anas (may Allah be pleased with him) reported that the Messenger of Allah (peace and blessings be upon him) said, “These mosques are not fit for any of this urine or filth; they are only for the remembrance of Allah Most High and the recitation of the Quran,” or words to that effect. Reported by Muslim.

[Commentary]
The author, may Allah have mercy on him, devoted this chapter in his Riyāḍ al-Ṣāliḥīn to explain the obligation of keeping mosques free from harm, filth, phlegm, spittle, and the like, and then he cited the hadith