Muwatta Malik, Book 14, Hadith 4 (USC-MSA) ⓘ

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, saw spittle on the wall of the qibla and scraped it off. Then he went up to the people and said, "Do not spit in front of you when you are praying, because Allah, the Blessed and Exalted, is in front of you when you pray."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، 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 تَبَارَكَ وَتَعَالَى قِبَلَ وَجْهِهِ إِذَا صَلَّى ‏" ‏ ‏.‏