It is narrated from Sayyiduna Rafi' bin Khadij (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said to Sayyiduna Bilal (may Allah be pleased with him): "Lead the Fajr prayer when it has become light enough that the people can see where their arrows fall."
Hadith Referenceالمعجم الكبير للطبراني / باب الراء / 4399