The Messenger of Allah (peace and blessings of Allah be upon him) saw some spittle on the wall of the mosque in the direction of the qiblah, so he scratched it off then turned to the people and said:
“If any one of you is praying, let him not spit in front of him, for indeed Allah is in front of him when he prays.”
Bukhari (406) and Muslim (547)
