Musnad Ahmad 1065:17

Ishaq ibn Isa narrated to us, Abd al-Rahman ibn Zayd ibn Aslam narrated to us, from his father, from Mahmud ibn Labid al-Ansari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Perform the dawn prayer (Fajr) when it is bright, for it is greater in reward."