Musnad Ahmad 27:1292

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, Isra'il 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, twenty-four or twenty-five times reciting in the two rak'ahs before the dawn prayer and the two rak'ahs after the sunset prayer: "Say, 'O disbelievers'" and "Say, 'He is Allah, [who is] One.'"