Musnad Ahmad 31:876

Abu Kamil narrated to us, Hammad narrated to us, he said: Abu Na'amah al-Sa'di narrated to us, Abu Nadrah narrated to us, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in prayer one day. When he was in part of his prayer, he took off his sandals and placed them to his left. When the people saw that, they took off their sandals. When he finished his prayer, he said: "What is the matter with you that you cast off your sandals?" They said: "We saw you cast off your sandals, so we cast off our sandals." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Gabriel came to me and informed me that there was filth—or he said: harm—in them, so I cast them off. So if one of you comes to the mosque, let him look at his sandals; if he sees filth—or he said: harm—in them, let him wipe them and pray in them." My father said: "It did not come in this hadith an explanation of what was on the sandals."