Sahih Muslim Book 4, Hadith Number 1116.

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

Abdullah b. Umar reported: The Messenger of Allah (may peace be upon him) saw spittle on the wall towards Qibla, and scratched it away and then turning to the people said: When any one of you prays, he must not spit in front of him, for Allah is in front of him when he is engaged in prayer.