Musnad Ahmad 1060:57

'Abd al-Rahman ibn Mahdi narrated to us, from Za'idah ibn Qudamah, from Mansur, from Hilal ibn Yasaf, from al-Rabi' ibn Khuthaym, from 'Amr ibn Maymun, from 'Abd al-Rahman ibn Abi Layla, from a woman of the Ansar, from Abu Ayyub, from the Prophet (peace and blessings of Allah be upon him), who said: "Is one of you unable to recite a third of the Qur'an in a night? For whoever recites 'Say, He is Allah, [who is] One, Allah, the Eternal Refuge' in a night has indeed recited a third of the Qur'an that night."