Musnad Abu Ya'la al-Mawsili 150:365

Muhammad ibn al-Minhal narrated to us, Yazid ibn Zuray' narrated to us, Rawh narrated to us, from Abdullah ibn Sam'an, from Sa'id ibn Abi Sa'id al-Maqburi, from al-Qa'qa' ibn Hakim, from Aisha, she said: I asked the Prophet, peace and blessings of Allah be upon him, about a man who steps with his sandals in filth. He said: "The soil is a purifier for them."