Sahih Bukhari Volume 8, Book 78, Hadith 6113

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 ‏"‏‏.‏

Narrated By 'Abdullah bin 'Umar : While the Prophet 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."