Musannaf Ibn Abi Shayba 3:6191

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Al-A’mash narrated to us, from Abu Wa’il, from Abdullah, who said: "Take care of these Mushafs (copies of the Quran), for they are more prone to escaping from the chests of men than camels are from their hobbles. So let not one of you say, 'I forgot such and such an ayah.' The Messenger of Allah (ﷺ) said: 'Rather, he was made to forget.'"