Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in prayer one day, and he took off his shoes and placed them on his left side. When the companions saw that the Messenger of Allah (peace and blessings be upon him) had taken off his shoes, they also took off their shoes. When the Prophet (peace and blessings be upon him) finished, he asked them: "What made you take off your shoes?" They replied, "O Messenger of Allah (peace and blessings be upon him), we saw you take off your shoes, so we also took off our shoes." The Prophet (peace and blessings be upon him) said: "(I only took them off) because a visitor (an angel) came to me and informed me that there was impurity on my shoes, so I took them off. Therefore, when any one of you enters the mosque, he should look at his shoes. If he sees impurity on them, he should rub them against the ground, and then he may pray in them."