Asim bin Umar bin Qatadah narrates from his father, from his grandfather, 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 for you. (The narrator is unsure; perhaps the words are:) It brings a greater reward."
Imam al-Bazzar recorded it, and its narrators are "thiqat (trustworthy)".
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1770