Musnad Ahmad 26:73

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Shaqiq, from Abdullah, who said: "Maintain these Mushafs"—and perhaps he said: "the Quran"—"for it is more prone to escaping from the chests of men than a camel from its tether." He said: And the Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not one of you say: 'I forgot such and such an ayah,' rather, he has been made to forget."