Sheikh `Abd al-Rahmân al-`Ajlân, lecturer at the Grand Mosque in Mecca
It is preferrable, according to the Sunnah, to have two calls made for Fajr prayer. The first of these should be offered before the time of the Fajr prayer and the other at the time when the Fajr Prayer comes in.
The Prophet (peace be upon him) said: “Bilâl calls (the adhân) while it is still night, so eat and drink until you hear the calling of Ibn Umm Maktûm.”
The narrator of the hadîth said: “Ibn Um Maktûm was blind, so he would not make the call to prayer until someone informed him that morning had arrived.
Source: Islam Today