Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2954
Ibn 'Umar rep rte that Allah's Messenger (may peace be upon him) combined the sunset and 'Isha ' prayers at Muzdalifa. He observed three rak'ahs of the sunset prayer and two rak'ahs of the 'Isha' prayer with one Iqama.