Sahih al-Bukhari 6111
Narrated `Abdullah bin `Umar:
While the Prophet (ﷺ) was praying, he saw sputum (on the wall) of the mosque, in the direction of the 
Qibla, and so he scraped it off with his hand, and the sign of disgust (was apparent from his face) and 
then said, "Whenever anyone of you is in prayer, he should not spit in front of him (in prayer) because 
Allah is in front of him."
حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، حَدَّثَنَا جُوَيْرِيَةُ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ بَيْنَا النَّبِيُّ صلى الله عليه وسلم يُصَلِّي رَأَى فِي قِبْلَةِ الْمَسْجِدِ نُخَامَةً، فَحَكَّهَا بِيَدِهِ، فَتَغَيَّظَ ثُمَّ قَالَ ‏
"‏ إِنَّ أَحَدَكُمْ إِذَا كَانَ فِي الصَّلاَةِ فَإِنَّ اللَّهَ حِيَالَ وَجْهِهِ، فَلاَ يَتَنَخَّمَنَّ حِيَالَ وَجْهِهِ فِي الصَّلاَةِ ‏"‏‏.‏