When the Mu'adh-dhin pronounced the Adhan for Fajr prayer and the dawn became evident the Prophet ordered a two rakʿat light prayer (Sunna) before the Iqama of the compulsory (congregational) prayer.  

أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ إِذَا اعْتَكَفَ الْمُؤَذِّنُ لِلصُّبْحِ وَبَدَا الصُّبْحُ صَلَّى رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ تُقَامَ الصَّلاَةُ  

The Prophet ﷺ used to offer two light rakʿat between the Adhan and the Iqama of the Fajr prayer.  

عَنْ عَائِشَةَ كَانَ النَّبِيُّ ﷺ يُصَلِّي رَكْعَتَيْنِ خَفِيفَتَيْنِ بَيْنَ النِّدَاءِ وَالإِقَامَةِ مِنْ صَلاَةِ الصُّبْحِ  

Messenger of Allah ﷺ said, "Bilal pronounces the Adhan at night, so keep on eating and drinking (Suhur) till Ibn Um Maktum pronounces the Adhan."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِنَّ بِلاَلاً يُنَادِي بِلَيْلٍ فَكُلُوا وَاشْرَبُوا حَتَّى يُنَادِيَ ابْنُ أُمِّ مَكْتُومٍ