It is narrated from Sayyiduna Abu Umamah or his brother (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) saw some people on whose heels a spot the size of a dirham or a fingernail had not been reached by water, and he (peace and blessings be upon him) said: "Woe to the heels from the Fire." The narrator says that each one of them would look at his heels, and if water had not reached them, he would repeat his ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 398