Sunan Abu Dawood, Chapter: Prayer (Kitab Al-Salat)

While the Apostle of Allah (peace_be_upon_him) was leading his Companions in prayer, he took off his sandals and laid them on his left side; so when the people saw this, they removed their sandals. When the Apostle of Allah (peace_be_upon_him) finished his prayer, he asked: What made you remove your sandals? The replied: We saw you remove your sandals, so we removed our sandals. The Apostle of Allah (peace_be_upon_him) then said: Gabriel came to me and informed me that there was filth in them. When any of you comes to the mosque, he should see; if he finds filth on his sandals, he should wipe it off and pray in them.