The command to keep refreshing one’s knowledge of the Qur’an and that it is disliked to say I have forgotten such-and-such a verse, but it is permissible to say I have been caused to forget Sahih Muslim 791
Translation
Abu Musa al-Ash'ari reported Allah's Apostle (ﷺ) as saying
Keep refreshing your knowledge of the Qur'an, for I swear by Him in Whose Hand is the life of Mahammad that it is more liable to escape than camels which are hobbled.