بُلُوغُ الْمَرَامِ
Bulugh al-Maram 245
The Book of Prayer
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 [1]
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].
Sunnah.com reference: Book 2, Hadith 114