The Prophet ﷺ saw on the foot of one of us a spot the size of a dirham that had not been washed, and he said, "Woe to the heels from the Fire."
حدثنا أسود بن عامر حدثنا إسرائيل عن أبي إسحاق عن سعيد بن أبي كرب
رأى النبي ﷺ في رجل رجل منا مثل الدرهم لم يغسله فقال ويل للعقب من النار
No references are available.
Its isnad is sahih, and Saeed bin Abu Kurayb is reliable, and the hadith was previously mentioned at 14906.