Sahih Muslim Book 7, Hadith Number 2954.

Chapter : Returning from ‘Arafat to Muzdalifa and excellence of observing sunset and ‘Isha prayers together at Muzdalifa.

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.