From Sahih Muslim (Hadith 1225):
Abu Sa'id al-Khudri reported:The Apostle of Allah (ﷺ) saw sputum sticking to the Qibla of the mosque. He scratched it off with a pebble and then forbade spitting on the right side or in front, but (it is permissible) to spit on the left side or under the left foot
Reference: Muslim 1225