Sahih Bukhari Volume 001, Book 004, Hadith Number 181.

Narated By Usama bin Zaid : "When Allah's Apostle departed from 'Arafat, he turned towards a mountain pass where he answered the call of nature. (After he had finished) I poured water and he performed ablution and then I said to him, "O Allah's Apostle! Will you offer the prayer?" He replied, "The Musalla (place of the prayer) is ahead of you (in Al-Muzdalifa)."