Musannaf Ibn Abi Shayba 3:1168

Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Anas, that Abu Bakr recited Surat al-Baqarah in the morning prayer. When he finished, ‘Umar said to him, "The sun was about to rise." He replied, "If it had risen, it would not have found us heedless."