The Messenger of Allah (ﷺ) said: Spitting in a mosque is a sin, and its expiation is that it should be buried.
Sahih Muslim Book 5, Hadith 69
← Previous Hadith The Messenger of Allah (ﷺ) said: When any one of you is engaged in prayer, he is holding intimate conversation with his Lord, so none of you must spit in front of him, or towards his right side, but towards his left side under his foot.

Sahih Muslim Book 5, Hadith 68 Next Hadith → I asked Qatada about spitting, in the mosque. He said: I heard Anas b. Malik say: I heard the Messenger of Allah (ﷺ) say: Spitting in the mosque is a sin, and its expiation is that it should be buried.

Sahih Muslim Book 5, Hadith 70