Sahih Muslim Book 7, Hadith Number 2952.

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

Ubaidullah b. ‘Abdullah b. ‘Umar reported on the authority of his father (Allah be pleased with them) that Allah’s Messenger (may peace be upon him) combined the sunset and ‘Isha, prayers at Muzdalifa and there was no prostration (i.e. any rak’ahs of Sunan or Nawafil prayers) in between them. He observed three rak’ahs of the sunset prayer and two rak’ahs of the ‘Isha prayer, and ‘Abdullah (b. ‘Umar) observed the prayers in this very manner (at Muzdalifa) until he met his Lord.