The Prophet ﷺ said, "A faithful believer while in prayer is speaking in private to his Lord, so he should neither spit in front of him nor to his right side but he could spit either on his left or under his foot."  

قَالَ النَّبِيُّ ﷺ إِنَّ الْمُؤْمِنَ إِذَا كَانَ فِي الصَّلاَةِ فَإِنَّمَا يُنَاجِي رَبَّهُ فَلاَ يَبْزُقَنَّ بَيْنَ يَدَيْهِ وَلاَ عَنْ يَمِينِهِ وَلَكِنْ عَنْ يَسَارِهِ أَوْ تَحْتَ قَدَمِهِ  

The Prophet ﷺ saw sputum on (the wall of) the mosque in the direction of the Qibla and scraped it off with gravel. Then he forbade Spitting in front or on the right, but allowed it on one's left or under one's left foot.  

أَنَّ النَّبِيَّ ﷺ أَبْصَرَ نُخَامَةً فِي قِبْلَةِ الْمَسْجِدِ فَحَكَّهَا بِحَصَاةٍ ثُمَّ نَهَى أَنْ يَبْزُقَ الرَّجُلُ بَيْنَ يَدَيْهِ أَوْ عَنْ يَمِينِهِ وَلَكِنْ عَنْ يَسَارِهِ أَوْ تَحْتَ قَدَمِهِ الْيُسْرَى وَعَنِ الزُّهْرِيِّ سَمِعَ حُمَيْدًا عَنْ أَبِي سَعِيدٍ نَحْوَهُ