Musannaf Ibn Abi Shayba 3:5513

Yazid ibn Harun narrated to us, from Hammad ibn Salamah, from Abu Na'amah, from Abu Nadrah, from Abu Sa'id al-Khudri, that the Messenger of Allah (may Allah bless him and grant him peace) prayed and took off his sandals, so the people took off their sandals. When he finished, he said, "Why did you take off your sandals?" They said, "O Messenger of Allah, we saw you take yours off, so we took ours off." He said, "Indeed, Gabriel (peace be upon him) came to me and informed me that there was filth on them. So when one of you comes to the mosque, let him turn his sandals over and look at them; if there is filth on them, let him wipe it off with the ground and pray in them."