Musnad Ahmad 33:1078

Aswad ibn Amir narrated to us, Isra'il narrated to us, from Abu Ishaq, from Sa'id ibn Abi Karb, from Jabir ibn Abdullah, who said: The Prophet, may Allah bless him and grant him peace, saw on the foot of a man among us [a spot] like a dirham that he had not washed. He said: "Woe to the heels from the Fire."