It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) never took off his shoes during prayer except once. When he (peace and blessings be upon him) took off his shoes, the people also took theirs off. He (peace and blessings be upon him) asked: "What is the matter with you (why did you take off your shoes)?" They said: You took yours off, so we took ours off. He (peace and blessings be upon him) said: "Jibril (peace be upon him) informed me that there was filth (or something harmful) on them." This hadith is sahih (authentic) on the condition of Imam Bukhari, since he used Abdullah bin al-Muthanna as evidence, but the two Shaykhs did not record it. Its supporting narration (shahid) is the well-known hadith of Maymun al-A'war.