Sahih Muslim , Book 5, Hadith 61 (In-Book) ⓘ

Abdullah b. Umar reported: The Messenger of Allah saw spittle on the wall towards Qibla, and scratched it away and then turning to the people said: When any one of you prays, he must not spit in front of him, for Allah is in front of him when he is engaged in prayer.

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى التَّمِيمِيُّ، قَالَ قَرَأْتُ عَلَى 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 قِبَلَ وَجْهِهِ إِذَا صَلَّى ‏" ‏ ‏.‏