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

A'bdullah (b. 'Umar) reported: I have never seen Allah's Messenger (ﷺ) but observing the prayers at their appointed times except two players, 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).

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