Musnad Ahmad 26:470

'Abd al-Razzaq narrated to us, Sufyan informed us, from Mansur, from Abu Wa'il, from Ibn Mas'ud, who attributed it to the Prophet, may Allah bless him and grant him peace, he said: "Maintain the Quran, for it is more prone to escaping from the chests of men than camels are from their hobbles. It is a wretched thing for one of them to say: 'I forgot such-and-such verse,' rather, he has been made to forget."