The Prophet (ﷺ) said, "Spitting in the mosque is a sin and its expiation is to bury it."
Sahih al-Bukhari Book 8, Hadith 65
← Previous Hadith The Prophet (ﷺ) saw sputum on (the wall of) the mosque in the direction of the Qibla and scraped it off with gravel. Then he forbade Spitting in front or on the right, but allowed it on one's left or under one's left foot.

Sahih al-Bukhari Book 8, Hadith 64 Next Hadith → Prophet said, "If anyone of you stands for prayer, he should not spit in front of him because in prayer he is speaking in private to Allah and he should not spit on his right as there is an angel, but he can spit either on his left or under his left foot and bury it (i.e. expectoration).

Sahih al-Bukhari Book 8, Hadith 66