Sunnah.com reference : Book 2, Hadith 114English translation : Book 2, Hadith 243Arabic reference : Book 2, Hadith 245
Narrated Anas (RA):
Allah's Messenger (ﷺ) said: "When anyone of you is engaged in Salat (prayer), he is holding intimate conversation with his Rabb so he should not spit in front of him, nor to his right side, but to his left side, under his foot".

[Agreed upon].
وَعَنْ أَنَسٍ ‏- رضى الله عنه ‏- قَالَ : قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ إِذَا كَانَ أَحَدُكُمْ فِي اَلصَّلَاةِ فَإِنَّهُ يُنَاجِي رَبَّهُ , فَلَا يَبْزُقَنَّ بَيْنَ يَدَيْهِ وَلَا عَنْ يَمِينِهِ , وَلَكِنْ عَنْ شِمَالِهِ تَحْتَ قَدَمِهِ }  مُتَّفَقٌ عَلَيْهِ  1‏‏1 ‏- صحيح .‏ رواه البخاري (1214)‏ ، ومسلم (551)‏ .‏