Sayyiduna Taliq (may Allah be pleased with him) narrates that the Noble Prophet (peace and blessings be upon him) said: The (true) Fajr is not that in which the light rises vertically in the horizon, rather it is that in which the red light spreads horizontally across the horizon.
Hadith Referenceالفتح الربانی / كتاب الصلاة / 1177