Musnad Abu Ya'la al-Mawsili 42:108

On the authority of Abu Sa’id, he said: The Messenger of Allah (may Allah bless him and grant him peace) entered the mosque and saw phlegm in the direction of the Qibla of the mosque. He cleared it with a stick or a reed that was with him. Then he turned to the people, and they recognized anger on his face. He said: "Who is the owner of this?" The people remained silent. The Messenger of Allah (may Allah bless him and grant him peace) said: "Would any of you like, if he stood in his place of prayer, for a man to face him and spit in his face?" They said: "O Messenger of Allah, we would not like that." He said: "For Allah is before you, so let none of you face anything of harm before him, but rather to his left or under his foot."