Sahih Bukhari Volume 2, Book 26, Hadith Number 728.

Narated By Usama bin Zaid : As soon as the Prophet departed from ‘Arafat, he went towards the mountain pass, and there he answered the call of) the prayer is ahead of you (i.e. at asked, “O Allah’s Apostle! Will you offer the prayer here?” He replied, “(The place of) the prayer is ahead of you (i.e. at Al-Muzdalifa).”