It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that once the Prophet (peace and blessings be upon him) saw phlegm stuck towards the qiblah in the mosque. The Prophet (peace and blessings be upon him) stood up and cleaned it, then turned towards the people and said: "When any one of you is in prayer, Allah is in front of his face, so none of you should clean his nose in front of his face during his prayer."