Hadith
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " الْبُصَاقُ فِي الْمَسْجِدِ خَطِيئَةٌ وَكَفَّارَتُهَا دَفْنُهَا " .
It was narrated that Anas said:"The Messenger of Allah (ﷺ) said: 'Spitting in the Masjid is a sin, and its expiation is to bury it
Sahih · Abu Ghuddah
Sunan an-Nasa'i 723 ·Book 8, Hadith 36·Also available on sunnah.com·