Sahih al-Bukhari 406Book 8, Hadith 58
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ أَخْبَرَنَا مَالِكٌ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ،. أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم رَأَى بُصَاقًا فِي جِدَارِ الْقِبْلَةِ فَحَكَّهُ، ثُمَّ أَقْبَلَ عَلَى النَّاسِ فَقَالَ " إِذَا كَانَ أَحَدُكُمْ يُصَلِّي، فَلاَ يَبْصُقْ قِبَلَ وَجْهِهِ، فَإِنَّ اللَّهَ قِبَلَ وَجْهِهِ إِذَا صَلَّى ".
Allah's Messenger (ﷺ) saw sputum on the wall of the mosque in the direction of the Qibla and scraped it off. He faced the people and said, "Whenever any one of you is praying, he should not spit in front of him because in the prayer Allah is in front of him."