Sahih al-Bukhari 1214
Narrated Anas:
The Prophet (ﷺ) said, "Whenever anyone of you is in prayer, he is speaking in private to his Lord and so 
he should neither spit in front of him nor on his right side but to his left side under his left foot."
حَدَّثَنَا مُحَمَّدٌ، حَدَّثَنَا غُنْدَرٌ، حَدَّثَنَا شُعْبَةُ، قَالَ سَمِعْتُ قَتَادَةَ، عَنْ أَنَسٍ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏
"‏ إِذَا كَانَ فِي الصَّلاَةِ فَإِنَّهُ يُنَاجِي رَبَّهُ، فَلاَ يَبْزُقَنَّ بَيْنَ يَدَيْهِ وَلاَ عَنْ يَمِينِهِ، وَلَكِنْ عَنْ شِمَالِهِ تَحْتَ قَدَمِهِ الْيُسْرَى ‏"‏‏.‏