Sunan Ibn Majah Book 1, Hadith 185
The Messenger of Allah said: 'Woe to the heels because of Hell-fire.'"
"The Messenger of Allah saw some people performing ablution, and their heels were dry. He said: 'Woe to the heels because of Hell-fire, perform ablution properly!'"
"Aishah saw 'Abdur-Rahman performing abluti0on, and she said: Perform ablution properly, for I heard the Messenger of Allah say: 'Woe to the Achilles' tendon because of Hell-fire.'"