Sunan Abi Dawud, Book 2, Hadith 475 (USC-MSA) ⓘ
Anas reported: The Messenger of Allah said: Spitting in the mosque is a sin and it is expiated by burying the spittle.
حَدَّثَنَا مُسَدَّدٌ، 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 475 (DarusSalam) & Hadith 475 (Hasan)