بُلُوغُ الْمَرَامِ
Bulugh al-Maram 262
The Book of Prayer
وَعَنْ أَنَسٍ - رضى الله عنه - قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ اَلْبُزَاقُ فِي اَلْمَسْجِدِ خَطِيئَةٌ وَكَفَّارَتُهَا دَفْنُهَا } مُتَّفَقٌ عَلَيْهِ . [1]
Narrated Anas (RA): Allah's Messenger (ﷺ) said: "Spitting in the mosque is a sin and its expiation is burying it." [Agreed upon].
Sunnah.com reference: Book 2, Hadith 134