It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was praying, when suddenly he took off his shoes. Seeing this, the people praying behind him also took off their shoes. When the Prophet (peace and blessings be upon him) finished the prayer, he asked the people: "Why did you take off your shoes?" The people replied: "O Messenger of Allah! When we saw you take off your shoes, we also took ours off." The Prophet (peace and blessings be upon him) said: "Jibril (peace be upon him) came to me and informed me that there was impurity on them (so I took off my shoes)." (Now the ruling regarding this is that) when a person comes to the mosque, he should turn his shoes over and look at them; if he sees any filth on them, he should clean it by rubbing it on the ground, and then he may pray in them.