Musannaf Ibn Abi Shayba 3:1306

Waki’ narrated to us, from Sufyan, from Ghaylan ibn Jami’ al-Muharibi, from ‘Amr ibn Maymun, who said: ‘Umar led us in the Fajr prayer while traveling, and he recited "Say, 'O disbelievers'" and "Say, 'He is Allah, [who is] One.'"