Anas bin Malik (May Allah be pleased with him) said:
The Messenger of Allah ﷺ said, "Spitting in the mosque is a sin, and its expiation is that the spittle should be buried in earth."

[Al-Bukhari and Muslim].

عن أنس رضي الله عنه أن رسول الله ﷺ : "البصاق في المسجد خطيئة، وكفارتها دفنها" ((متفق عليه)).