Sahih Muslim Book 7, Hadith Number 2942.

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: Allah’s Messenger (may peace be upon him) on his way back from ‘Arafat got down in one of these creeks (to answer the call of nature), and after he had done that I poured water (over his hands) and said: Are you going to pray? Thereupon he said: The place of prayer is ahead of you.