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 789 a
Translation
'Abdullah b. 'Umar reported Allah's Messenger (ﷺ) as saying
The example of a man who has memorised the Qur'an is like that of a hobbled camel. If he remained vigilant, he would be able to retain it (with him), and if he loosened the hobbled camel it would escape.