Sahih Muslim Book 4, Hadith Number 1118.

Chapter : It is forbidden to remove pebbles and smooth the ground while engaged in prayer.

Abu Sa’id al-Khudri reported: The Apostle of Allah (may peace be upon him) 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.