Sahih Muslim Book 7, Hadith Number 2953.
Chapter : Returning from ‘Arafat to Muzdalifa and excellence of observing sunset and ‘Isha prayers together at Muzdalifa.
Sa’id b. Jubair reported that he observed the sunset and ‘Isha prayers at Muzdalifa with (one) iqama. He narrated on the authority of Ibn ‘Umar (Allah be pleased with them) that he observed prayers like this and Ibn ‘Umar (Allah be pleased with them) narrated that Allah’s Apostle (may peace be upon him) did like this. Shu’ba reported this Hadith with the same chain of transmitters and said: He (the Holy Prophet) observed the two prayers (together) with one iqama.