فَإِذَا طَلَعَ الْفَجْرُ صَلَّى الْإِمَامُ بِالنَّاسِ الْفَجْرَ بِغَلَسٍ.
Translation: When dawn appears, the imam leads Fajr while it is still dark early in the dawn period.
Ghalas means the remaining darkness of early dawn, after true dawn has begun. It never means offering Fajr before the prayer time has entered.
On the morning of 10 Dhu al-Hijjah the pilgrims perform Fajr, then proceed to the standing and supplication at Muzdalifah.