Sahih Muslim Book 4, Hadith Number 1121.

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

Abu Huraira reported that the Messenger of Allah (may peace be upon him) saw some sputum in the direction of the Qibla of the mosque. He turned towards people and said: How Is it that someone amongst you stands before his Lord and then spits out in front of Him? Does any one of you like that he should be made to stand in front of someone and then spit at his face? So when any one of you spits, he must spit on his left side under his foot. But if he does not find (space to spit) he should do like this. Qasim (one of the narrators) spat in his cloth and then folded it and rubbed it.