Musnad Abu Ya'la al-Mawsili 42:134

Ishaq narrated to us, Abu Khalid narrated to us, from al-A’mash, from al-Dahhak, from Abu Sa’id, from the Prophet (peace be upon him). And al-A’mash, from Hilal ibn Yasaf, from Ibn Abi Layla, and al-A’mash, from Ibrahim, who said: The Messenger of Allah (peace be upon him) said: "Is one of you unable to recite a third of the Qur'an in a night?" They said: "Who can endure that?" He said: "Recite 'Say, He is Allah, [who is] One' (Surah al-Ikhlas), for it is a third of the Qur'an."