Hadith 504610
Chapter 14: The Qibla - كتاب القبلة
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 " .
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."
Reference Muwatta Malik 504610
USC-MSA web (English) reference : Book 14, Hadith 4