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]. (ID)
App reference (ID): Book 18, Hadith 183 (ID)
Arabic/English book reference (ID): Book 18, Hadith 1693 (ID)