The Difference between the First Athan of Fajr and the Second

Question :

I have read that the wording As-salatu khairun minan- nawm (Prayer is better than sleep) should be recited in the first Athan of Fajr, but in our time, we hear it in the second Athan. We request an explanation with evidence.

Answer:

This sentence is pronounced in the Athan of Fajr, and that is the call which is made after the time for Fajr begins, for the performance of the obligatory prayer. As for the Hadiths which have been reported stating that it was in the first Athan, they are correct. But what is meant by the first, is that it is the Athan which is pronounced from the minaret, when the time begins. And what is meant by the second, is the Iqamah, because it is also referred to as an "Athan" as in his saying:
«Between every two Athans there is a prayer.

So this means between the Athan and the Iqamah.

As for the Athan which is called at the end of the night, the most correct opinion is that it was something special for the month of Ramadhan, based upon the Hadith:
"Do not let the Athan of Bilal prevent you from your Sahur because it is only called during the night to awaken those of you who are sleeping and so that those of you who are standing in prayer may return.

Thus, it is clear that it is performed in order to wake the sleepers for Sahur and so that the one who is standing in prayer may return, knowing that the time for Sahur is near and that he must finish his prayer. And there is no need to mention therein that prayer is better than sleep.

Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 2 Pages 132-133