Sahih al-Bukhari 412
Sahih al-Bukhari · Book 8: Prayers (Salat) · It is forbidden to spit on the right side while in Salat (prayers)
Narrated Anas:
حَدَّثَنَا حَفْصُ بْنُ عُمَرَ ، قَالَ حَدَّثَنَا شُعْبَةُ ، قَالَ أَخْبَرَنِي قَتَادَةُ ، قَالَ سَمِعْتُ أَنَسًا ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم " لاَ يَتْفِلَنَّ أَحَدُكُمْ بَيْنَ يَدَيْهِ وَلاَ عَنْ يَمِينِهِ، وَلَكِنْ عَنْ يَسَارِهِ أَوْ تَحْتَ رِجْلِهِ " .
The Prophet (ﷺ) said, "None of you should spit in front or on his right but he could spit either on his left or under his foot."