ENOfficial
It was narrated from Abu Sa'eed Al-Khudri that the Prophet (ﷺ) saw some spittle in the Qiblah of the Masjid. He scratched it off with a pebble and forbade a man to spit to his front or to his right. He said: "Let him spit to his left or beneath his left foot."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، أَنَّ النَّبِيَّ صلى الله عليه وسلم رَأَى نُخَامَةً فِي قِبْلَةِ الْمَسْجِدِ فَحَكَّهَا بِحَصَاةٍ وَنَهَى أَنْ يَبْصُقَ الرَّجُلُ بَيْنَ يَدَيْهِ أَوْ عَنْ يَمِينِهِ وَقَالَ " يَبْصُقُ عَنْ يَسَارِهِ أَوْ تَحْتَ قَدَمِهِ الْيُسْرَى " .
sunnah.com