The Prophet ﷺ said, "Spitting in the mosque is a sin." He was asked, "Is it really so?" He replied, "Yes, and its expiation is to bury it."
حدثنا حجاج قال سمعت شعبة يحدث قال قلت لقتادة أسمعت
عن النبي ﷺ أنه كان يقول البصاق في المسجد خطيئة قال نعم وكفارته دفنه
No references are available.
Its isnad is sahih, previously mentioned in 12001.