Musnad Ahmad 28:333

Waki’ narrated to us, Sufyan and ‘Abd al-Rahman narrated to us, from Sufyan, from Mansur, from Hilal ibn Yasaf, from Abu Yahya, from ‘Abdullah ibn ‘Amr, who said: The Messenger of Allah (peace and blessings of Allah be upon him) saw some people performing ablution while their heels were glistening [dry]. He said: "Woe to the heels from the Fire. Perform the ablution thoroughly."