Sahih Muslim
The Book of Pilgrimage (Kitab Al-Hajj)
Book Number: 007 / Hadith Number: 2942
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.