Sahih Muslim Book 7, Hadith Number 2956.

Chapter : Excellence of observing the dawn prayer at the earliest part of the dawn on the 10th of Dhu’l-Hijja at Muzdalifa.

A’bdullah (b. ‘Umar) reported: I have never seen Allah’s Messenger, (may peace be upon him) but observing the prayers at their appointed times except two players, sunset and ‘Isha, at Muzdalifa (where he deferred the sunset prayer to combine it with ‘Isha and he observed the dawn prayer before its stipulated time on that day (10th of Dhu’l-Hijja).