Abu Dawud
Purification (Kitab Al-Taharah)
Book Number: 001 / Hadith Number: 98
Narated By 'Abd Allah b. 'Amr : The Messenger of Allah (pbuh) saw some people (performing ablution) while their heels were dry. Hen then said: Woe to the heels because of Hell. Perform the ablution in full.