Sahih Muslim Book 2, Hadith Number 466.

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

Salim, the freed slave of Mahri, reported: I and ‘Abd al-Rahman b. Abu Bakr went out (in order to join) the funeral procession of Sa’d b. Abi Waqqas and passed by the door of the residence of ‘Aisha, and then he transmitted a Hadith like this from her who (narrated it) from the Holy Prophet (may peace be upon him).