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