Is it permissible to pray Fajr immediately upon hearing the Adhan?

One can rely on one of the following methods to ascertain the entry of prayer time: 1. Verifying the appearance of true dawn (Al-Fajr Al-Sadiq) by direct sighting, if possible. 2. Hearing the Adhan (call to prayer) if the Mu'adhdhin (caller to prayer) is trustworthy and calls the Adhan upon the advent of dawn. 3. A reliable prayer timetable that specifies prayer times.

One must not begin the prayer until after verifying that its time has entered.