Musannaf Ibn Abi Shayba 1:272

Ali ibn Mushir narrated to us, from Layth, from Abd al-Rahman ibn Sabit, from Abu Umamah, or from his brother, who said: The Messenger of Allah (may Allah bless him and grant him peace) saw some people performing ablution, and he saw the heel of one of them exposed, which the water had not reached. So the Messenger of Allah (may Allah bless him and grant him peace) said: "Woe to the heels from the Fire."