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 788 b
Translation
'A'isha reported that the Messenger of Allah (ﷺ) listened to the recitation of the Qur'an by a man in the mosque. Thereupon he said
May Allah have mercy upon him; be reminded me of the verse which I had been made to forget.