The Prophet (peace and blessings be upon him) said:
"Perform the Fajr prayer when the dawn has become well lit, for there is a very great reward in it."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 970
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[ت ن حب] عن رافع. المشكاة ٦١٤، الإرواء ٢٥٨.