Sahih Muslim Book 4, Hadith Number 1126.

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

Abu Dharr reported: The Apostle of Allah (may peace be upon him) said: The deeds of my people, good and bad, were presented before me, and I found the removal of something objectionable from the road among their good deeds, and the sputum mucus left unburied in the mosque among their evil deeds.