Sahih Muslim , Book 5, Hadith 69 (In-Book) ⓘ
Anas b. Malik reported: The Messenger of Allah said: Spitting in a mosque is a sin, and its expiation is that it should be buried.
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَقُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ يَحْيَى أَخْبَرَنَا وَقَالَ، قُتَيْبَةُ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ قَتَادَةَ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الْبُزَاقُ فِي الْمَسْجِدِ خَطِيئَةٌ وَكَفَّارَتُهَا دَفْنُهَا " .
also referenced under: Hadith 552 (Reference) & Book 4, Hadith 1124 (USC-MSA)