It is narrated from Sayyiduna Rafi' bin Khadij (may Allah be pleased with him): I heard the Messenger of Allah (peace and blessings be upon him) say: "Offer Fajr when it has become light, for in it there is greater reward."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2151