Sayyiduna Bilal (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Offer the Fajr prayer when it has become light, for that brings a greater reward."
Imam al-Bazzar and al-Tabarani in al-Mu'jam al-Kabir recorded it; its chain contains Ayyub bin Sayyar, who is "da'if (weak)".
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1769