مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 708
Prayer
Mosques and places of Prayer - Section 1
وَعَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «الْبُزَاقُ فِي الْمَسْجِد خَطِيئَة وكفارتها دَفنهَا»
Anas reported God's Messenger as saying, “Spitting in the mosque is a sin which is expiated by burying the spittle.” (Bukhari and Muslim.)
Mishkat al-Masabih, 708مُتَّفَقٌ عَلَيْهِ (الألباني)