The Prophet ﷺ scraped off some spittle from the wall of the mosque.
حدثنا وكيع قال حدثنا هشام عن أبيه
أن النبي ﷺ حك بزاقا في المسجد
No references are available.
Its isnad is sahih. It was mentioned previously at length many times, and it is found in Bukhari 1/509, no. 407.