Sunan an-Nasa'i, Volume 1, Book 8, Hadith 724 (DarusSalam) ⓘ
It was narrated that Anas said: "The Messenger of Allah said: 'Spitting in the Masjid is a sin, and its expiation is to bury it.'"
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الْبُصَاقُ فِي الْمَسْجِدِ خَطِيئَةٌ وَكَفَّارَتُهَا دَفْنُهَا " .
also referenced under: Hadith 723 (Reference) & Book 8, Hadith 36 (In-Book)