The Chapter of the Qibla
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 43
وَ رَأَى ص نُخَامَةً فِي اَلْمَسْجِدِ فَمَشَى إِلَيْهَا بِعُرْجُونٍ مِنْ عَرَاجِينِ اِبْنِ طَابٍ فَحَكَّهَا ثُمَّ رَجَعَ اَلْقَهْقَرَى فَبَنَى عَلَى صَلاَتِهِ وَقَالَ اَلصَّادِقُ عَلَيْهِ اَلسَّلاَمُ «وَ هَذَا يَفْتَحُ مِنَ اَلصَّلاَةِ أَبْوَاباً كَثِيرَةً.
The Prophet صلى الله عليه وعلى آله وسلم saw phlegm (or a spittle) in the mosque. He walked over to it with a palm branch from the branches of Ibn Tab and scraped it off, then returned backward and continued his prayer. And Imam Jafar ibn Muhammad Al-Sadiq عليه السلام said: "This opens many doors regarding (permissible) actions during prayer."
- Ṣaḥīḥ al-Faqīh