The Prophet (peace and blessings be upon him) said:
"Perform the Fajr prayer in the light of dawn, until the people can see (in the light) the places where their arrows fall."
Hadith Referenceصحیح الجامع الصغیر / حرف الألف / 969
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[الطيالسي] عن رافع بن خديج. الإرواء ٢٥٨.