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. (Using translation from Bukhārī 618)
أَنَّ رَسُولَ اللهِ ﷺ كَانَ «إِذَا سَكَتَ الْمُؤَذِّنُ لِأَذَانِ الصُّبْحِ وَبَدَا الصُّبْحُ صَلَّى رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ تُقَامَ الصَّلَاةُ»