Musnad Ahmad 27:461

'Abd al-Razzaq narrated to us, Sufyan informed us, from Abu Ishaq, from Mujahid, from Ibn 'Umar, who said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) more than twenty-five times—or more than twenty times—'Abd al-Razzaq said: "And I am in doubt"—reciting in the two rak'ahs of the dawn prayer: "Say, 'O disbelievers'" and "Say, 'He is Allah, [who is] One.'"