Musannaf Ibn Abi Shayba 22:83

Ibn ‘Uyaynah narrated to us, from Mansur, from Abu Wa’il, who said: ‘Abdullah said: "Keep renewing this Qur’an, for it is more prone to escaping from the hearts of men than camels [are from their tether]." He said: And the Messenger of Allah (ﷺ) said: "It is a wretched thing for one of them to say: 'I forgot such-and-such verse.' Rather, he has been made to forget."