Musnad Abu Ya'la al-Mawsili 154:90

Muhammad ibn al-Minhal, the brother of al-Hajjaj, narrated to us, saying: Abd al-Wahid, meaning ibn Ziyad, narrated to us from Layth, who said: Abu Muhammad narrated to me, saying: I watched Ibn Umar for a month, and I heard him in the two rak'ahs before the morning prayer reciting "Say, 'O disbelievers'" and "Say, 'He is Allah, [who is] One.'" He said: So I mentioned that to him, and he said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) for a month or twenty-five days reciting in the two rak'ahs before the morning prayer "Say, 'O disbelievers'" and "Say, 'He is Allah, [who is] One.'" And he said: Indeed, one of them is equivalent to a third of the Qur'an, and the other is equivalent to a quarter of the Qur'an. "Say, 'He is Allah, [who is] One'" is equivalent to a third of the Qur'an, and "Say, 'O disbelievers'" is equivalent to a quarter of the Qur'an.