Musnad Abu Ya'la al-Mawsili 42:221

Zuhayr narrated to us, Abd al-Rahman narrated to us, Hammad ibn Salamah narrated to us, 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) took off his sandals during prayer and placed them to his left. So they took off their sandals. He said, "What is the matter with you?" They said, "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, let him look; if he sees filth or harm on them, let him wipe them, then let him pray in them."