Sahih Muslim Book 7, Hadith Number 2946.
Chapter : Returning from ‘Arafat to Muzdalifa and excellence of observing sunset and ‘Isha prayers together at Muzdalifa.
Usama b. Zaid (Allah be pleased with him) reported that he sat behind Allah’s Messenger (may peace be upon him) on his ride as he came back from ‘Arafa. And as he came to the valley, he halted his camel, and then went to the wilderness (to urinate). And when he came back, I poured water on him from the jug and he performed ablution, and then rode on until he came to Muzdalifa and there he combined the sunset and ‘Isha prayers.