Musannaf Ibn Abi Shayba 3:865

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Muhammad ibn ‘Ajlan, from ‘Asim ibn ‘Umar ibn Qatadah, from Mahmud ibn Labid, from Rafi‘ ibn Khadij, he 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 it is greater in reward."