Sahih al-Bukhari, Book 8, Hadith 65 (In-Book) ⓘ
Narrated Anas bin Malik: The Prophet said, "Spitting in the mosque is a sin and its expiation is to bury it."
حَدَّثَنَا آدَمُ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنَا قَتَادَةُ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم " الْبُزَاقُ فِي الْمَسْجِدِ خَطِيئَةٌ، وَكَفَّارَتُهَا دَفْنُهَا " .
also referenced under: Hadith 415 (DarusSalam) & Volume 1, Book 8, Hadith 407 (USC-MSA)