كتاب الحج
15  
The Book of Pilgrimage
(48)
Chapter: It is recommended to pray Subh when it is still dark on the day of sacrifice in Al-Muzdalifah, and to do it very early after ascertaining that dawn has broken
(48)
باب اسْتِحْبَابِ زِيَادَةِ التَّغْلِيسِ بِصَلاَةِ الصُّبْحِ يَوْمَ النَّحْرِ بِالْمُزْدَلِفَةِ وَالْمُبَالَغَةِ فِيهِ بَعْدَ تَحَقُّقِ طُلُوعِ الْفَجْرِ
Sahih Muslim 1289 a

A'bdullah (b. 'Umar) reported:

I have never seen Allah's Messenger, (ﷺ) but observing the prayers at their appointed times except two prayers, 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).
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ وَأَبُو كُرَيْبٍ جَمِيعًا عَنْ أَبِي مُعَاوِيَةَ، - قَالَ يَحْيَى أَخْبَرَنَا أَبُو مُعَاوِيَةَ، - عَنِ الأَعْمَشِ، عَنْ عُمَارَةَ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ يَزِيدَ، عَنْ عَبْدِ اللَّهِ، قَالَ مَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم صَلَّى صَلاَةً إِلاَّ لِمِيقَاتِهَا إِلاَّ صَلاَتَيْنِ صَلاَةَ الْمَغْرِبِ وَالْعِشَاءِ بِجَمْعٍ وَصَلَّى الْفَجْرَ يَوْمَئِذٍ قَبْلَ مِيقَاتِهَا ‏.‏
Reference : Sahih Muslim 1289a
In-book reference : Book 15, Hadith 320
USC-MSA web (English) reference : Book 7, Hadith 2956
  (deprecated numbering scheme)