Sahih Muslim Book 7, Hadith Number 2955.

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

Sa’id b. Jubair reported: We came back along with Ibn ‘Umar till we reached Muzdalifa. There he led us in the sunset and ‘Isha prayers with one iqama and we then proceeded and he said: This is how Allah’s Messenger (may peace be upon him) led us in prayer at this place.