Sahih Muslim Book 002, Hadith Number 0468.
Chapter : The washing of feet properly is indispensable in Wudu.
'Abdullah b. 'Amr reported: We returned from Mecca to Medina with the Messenger of Allah (may peace be upon him), and when we came to some water on the way, some of the people were in a hurry at the time of the afternoon prayer and performed ablution hurriedly; and when we reached them, their heels were dry, no water had touched them. The Prophet (may peace be upon him) said: Woe to (dry) heels, because of Hell-fire. Make your ablution thorough.