Sahih Muslim Book 7, Hadith Number 2944.

Chapter : Returning from ‘Arafat to Muzdalifa and excellence of observing sunset and ‘Isha prayers together at Muzdalifa.

Kuraib reported that he asked Usama b. Zaid (Allah be pleased with him) What did you do in the evening of ‘Arafa as you rode behind Allah’s Messenger (may peace be upon him)? He said: We came to a valley where people generally halted their (camels) for the sunset prayer. Allah’s Messenger (may peace be upon him) halted his camel and urinated (and he did not say that he had poured water). He then called for water and performed light ablution. I said: Messenger of Allah, the prayer! Thereupon he said: Prayer awaits you (at Muzdalifa) and he rode on until we came to Muzdalifa. Then he offered the sunset prayer and the people halted their camels at their places, and did not untie them until Iqama was pronounced for the ‘Isha prayer and he observed the prayer, and then they untied (their camels). I said: What did you do in the morning? He said: Al-Fadl b. Abbas (Allah be pleased with them) sat behind him (the Holy Prophet) in the morning, whereas I proceeded on foot with the Quraish who had gone ahead.