Sahih al-Bukhari 413
Narrated Anas bin Malik:
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."
حَدَّثَنَا آدَمُ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنَا قَتَادَةُ، قَالَ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏
"‏ إِنَّ الْمُؤْمِنَ إِذَا كَانَ فِي الصَّلاَةِ فَإِنَّمَا يُنَاجِي رَبَّهُ، فَلاَ يَبْزُقَنَّ بَيْنَ يَدَيْهِ وَلاَ عَنْ يَمِينِهِ، وَلَكِنْ عَنْ يَسَارِهِ أَوْ تَحْتَ قَدَمِهِ ‏"‏‏.‏