Sayyiduna Mahmud bin Labid al-Ansari (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 Ahmad recorded it; its chain contains Abdul Rahman bin Zayd bin Aslam, who is "da'if (weak)".
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1766