Musnad Ahmad 27:1241

Abu Ahmad al-Zubayri narrated to us, Sufyan narrated to us, from Abu Ishaq, from Mujahid, from Ibn Umar, who said: I observed the Prophet, peace and blessings of Allah be upon him, for a month, and he used to recite in the two rak'ahs before the dawn prayer: "Say, 'O disbelievers,'" and "Say, 'He is Allah, [who is] One.'"