Sahih Muslim Book 2, Hadith Number 464.

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

Salim, the freed slave of Shaddad, said: I came to ‘Aisha, the wife of the Holy Prophet (may peace be upon him), on the day when Sa’db. Abi Waqqas died. ‘Abd al-Rahman b. Abu Bakr also came there and he performed ablution in her presence. She (Hadrat ‘Aisha) said: Abd al-Rahman, complete the ablution as I heard the Allah’s Messenger (may peace be upon him) say: Woe to the heels because of hell-fire.