Musnad Ahmad 26:738

'Abd al-Razzaq narrated to us, Ibn Jurayj informed us, 'Abdah ibn Abi Lubabah narrated to me, that Shaqiq ibn Salamah said: I heard Ibn Mas'ud saying: I heard the Prophet, peace and blessings of Allah be upon him, saying: "It is a wretched thing for a man, or for a person, to say: 'I forgot such-and-such a surah' or 'such-and-such an ayah.' Rather, he has been made to forget."