Sahih Muslim Book 007, Hadith Number 2941.
Chapter : Returning from 'Arafat to Muzdalifa and excellence of observing sunset and 'Isha prayers together at Muzdalifa.
Kuraib, the freed slave of Ibn Abbas, narrated from Usama b. Zaid (Allah be pleased with him) that he had heard him saying: Allah's Messenger (may peace be upon him) proceeded from 'Arafa, and as he approached the creek of a hill, he got down (from his camel) and urinated, and then performed a light ablution. I said to him: Prayer, whereupon he said: The prayer awaits you (at Muzdalifa). So he rode again, and as he came to Muzdalifa, he got down and performed ablution well. Then Iqima was pronounced for prayer, and he 'observed the sunset prayer. Then every person made his camel kneel down there, and then Iqama was pronounced for 'Isha prayer and he observed it, and he (the Holy Prophet) did not observe any prayer (either Sunan or Nawifil) in between them (He observed the Fard of sunset and 'Isha prayers successively.)