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 b

This hadith has been transmitted by Al-A`mash with a slight variation of words, i.e. he said before its time when it was still dark.

وَحَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ، وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ، جَمِيعًا عَنْ جَرِيرٍ، عَنِ الأَعْمَشِ، بِهَذَا الإِسْنَادِ وَقَالَ قَبْلَ وَقْتِهَا بِغَلَسٍ ‏.‏
Reference : Sahih Muslim 1289b
In-book reference : Book 15, Hadith 321
USC-MSA web (English) reference : Book 7, Hadith 2957
  (deprecated numbering scheme)