Sayyiduna Abu Umamah (may Allah be pleased with him) — or his brother — narrates that the Messenger of Allah (peace and blessings be upon him) saw some people, and on the heel of one of them there was a spot the size of a dirham or the size of a fingernail which the water had not reached, so he (peace and blessings be upon him) began to say twice: "Woe to the heels from Hellfire."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 8085