Musnad Ahmad 31:165

Yazid narrated to us, Hammad ibn Salamah informed us, from Abu Na'amah, from Abu Nadrah, from Abu Sa'id al-Khudri: That the Messenger of Allah (peace and blessings of Allah be upon him) 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 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 sandal over and look at it. If he sees filth on it, let him rub it against the ground, then let him pray in them."