Musnad Ahmad 3596

It was narrated that ‘Abdullah said: Keep refreshing your knowledge of these Mushafs - or he said, Qur’an - for it is more inclined to escape from the hearts of men than camels from their ropes. And the Messenger of Allah (ﷺ) said: “No one of you should say, I have forgotten such and such a verse; rather he has been caused to forget.”