The Book of the Masjids
8/30. Chapter:
Spitting In The Masjid
Sunan an-Nasai
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.'"
Reference : Sunan an-Nasa'i 723In-book reference : Book 8, Hadith 36English translation : Vol. 1, Book 8, Hadith 724