Musannaf Ibn Abi Shayba 3:876

Waki' narrated to us, from Hisham bin Sa'd, from Zayd bin Aslam, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Perform the dawn prayer (Fajr) when it is bright (isfar), for indeed, the more you brighten it, the greater the reward."