Sahih Muslim Book 7, Hadith Number 2945.

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 when Allah’s Messenger (may peace be upon him) came to the valley where the rich (people of Mecca) used to get down, he got down and urinated (and he did not mention about pouring water); he then called for water and performed a light ablution. I said: Messenger of Allah, the prayer I Thereupon he said: Prayer awaits you ahead.