Sahih Muslim 1289
Sahih Muslim · Book 15: The Book of Pilgrimage · 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
وَحَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ ، وَإِسْحَاقُ بْنُ إِبْرَاهِيمَ ، جَمِيعًا عَنْ جَرِيرٍ ، عَنِ الأَعْمَشِ ، بِهَذَا الإِسْنَادِ وَقَالَ قَبْلَ وَقْتِهَا بِغَلَسٍ .
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.