It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both): "Fajr (dawn) is of two kinds: the fajr that rises while it is still night, in which eating and drinking are permissible and prayer is not permissible; and the other fajr, in which prayer is permissible and eating and drinking are forbidden, and it is the one that spreads over the tops of the mountains."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1768