the Messenger of Allah said: "Do not let the Adhan of Bilal prevent you from your Sahar, nor the drawn out Fajr, but the Fajr that spreads on the horizon." (Using translation from Tirmidhī 706)
قَالَ رَسُولُ اللهِ ﷺ لَا يَمْنَعَنَّكُمْ مِنْ سُحُورِكُمْ أَذَانُ بِلَالٍ وَلَا الْفَجْرُ الْمُسْتَطِيلُ وَلَكِنِ الْفَجْرُ الْمُسْتَطِيرُ فِي الْأُفُقِ