Sahih Muslim Book 7, Hadith Number 2943.

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) narrated: Allah’s Messenger (may peace be upon him) was on his way back from ‘Arafat and as he reached the creek (of a hillock) he got down and urinated (Usama did not say that he poured water), but said: He (the Holy Prophet) called for water and performed ablution, but it was not a thorough one. I said: Messenger of Allah, the prayer ! Thereupon he said: Prayer awaits you ahead (at Muzdalifa). He then proceeded, until he reached Muzdalifa and observed sunset and ‘Isha prayers (together) there.