Musnad Abi Ya'la 1081 — the Messenger of Allah ﷺ entered the mosque

It is narrated from Sayyiduna Abu Sa'id (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) entered the mosque and his eye fell on some phlegm on the wall in the direction of the qiblah. He (peace and blessings be upon him) scraped it clean with a stick or twig that he had with him. Then he (peace and blessings be upon him) turned to the people in such a state that they could see the signs of anger on his (peace and blessings be upon him) blessed face. He (peace and blessings be upon him) asked: "Who is responsible for this?" Everyone remained silent. Then the Messenger of Allah (peace and blessings be upon him) said: "Would any of you like it if, when he stands at his place of prayer, someone comes in front of him and spits in his face?" The Companions said: O Messenger of Allah! We would never like that. He (peace and blessings be upon him) said: "Then remember that Allah Almighty is in front of you, so none of you should throw anything offensive in front of himself; rather (let him spit) to his left or under his foot."