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

Sahih Muslim · Book 15 · Hadith 1289 a

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 صَلا…

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 comb…

Sahih Muslim · Book 15 · Hadith 1289 b

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.