Should the Fajr prayer be performed at the time of the first Adhan or the second?

The Sunnah is for the Fajr prayer to have two adhans (calls to prayer): the first is at night, to wake up those who are asleep and alert those who are performing night prayers. No obligation of prayer or prohibition of food is associated with this first adhan. As for the second adhan, it is the one that obligates prayer and prohibits food for the fasting person, and its time is upon the entry of the Fajr prayer time. There is no difference between the Fajr prayer and the Subh prayer; they are one single prayer which is not valid except after the second adhan.