Sahih Muslim Book 2, Hadith Number 470.

Chapter : The washing of feet properly is indispensable in Wudu.

‘Abdullah b. Amr reported: The Messenger of Allah (may peace be upon him) lagged behind us on a journey. We travelled (back) and be took him; and then came the time of the afternoon prayer, and as we were going to wipe our feet he (the Holy Prophet) called out: Woe to the heels because of Hell-fire.