Sahih Muslim 549
Sahih Muslim · Book 5: The Book of Mosques and Places of Prayer · The prohibition of spitting in the masjid, during prayer and at other times. The prohibition of a praying person spitting in front of him or to his right
'A'isha reported:
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ ، عَنْ مَالِكِ بْنِ أَنَسٍ ، فِيمَا قُرِئَ عَلَيْهِ عَنْ هِشَامِ بْنِ عُرْوَةَ ، عَنْ أَبِيهِ ، عَنْ عَائِشَةَ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم رَأَى بُصَاقًا فِي جِدَارِ الْقِبْلَةِ أَوْ مُخَاطًا أَوْ نُخَامَةً فَحَكَّهُ .
The Apostle of Allah (may, peace be upon him) saw spittle or snot or sputum, sticking to the wall towards Qibla and scratched it off.