Musnad Abu Ya'la al-Mawsili 42:45

Abu Bakr narrated to us, Abu Khalid al-Ahmar narrated to us, from al-A'mash, from al-Dahhak al-Mashriqi, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Is one of you unable to recite a third of the Qur'an?" They said, "That is difficult for us, and who among us can do that?" He said: "Recite 'Say, He is Allah, [who is] One' (Surah al-Ikhlas), for it is equivalent to a third of the Qur'an."