Sahih Bukhari Volume 2, Book 26, Hadith Number 732.

Narated By Usama bin Zaid : Allah’s Apostle proceeded from ‘Arafat and dismounted at the mountainous pass and then urinated and performed a light ablution. I said to him, “(Shall we offer) the prayer?” He replied, “The prayer is ahead of you (i.e. at Al-Muzdalifa).” When he came to Al-Muzdalifa, he performed a perfect ablution. Then Iqama for the prayer was pronounced and he offended the Maghrib prayer and then every person made his camel kneel at his place; and then Iqama for the prayer was pronounced and he offered the (‘Isha’) prayer and he did not offer any prayer in between them (i.e. Maghrib and ‘Isha’ prayers).