Musnad Ahmad 457:47

'Abd al-Rahman—who is Ibn Mahdi—narrated to us, from Sufyan, from Abu Qays, from 'Amr ibn Maymun, from Abu Mas'ud, 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: 'Allah, the One, the Eternal Refuge'?"