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.