Sahih Muslim 788
Sahih Muslim · Book 6: The Book of Prayer - Travellers · 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
'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:
وَحَدَّثَنَا ابْنُ نُمَيْرٍ ، حَدَّثَنَا عَبْدَةُ ، وَأَبُو مُعَاوِيَةَ عَنْ هِشَامٍ ، عَنْ أَبِيهِ ، عَنْ عَائِشَةَ ، قَالَتْ كَانَ النَّبِيُّ صلى الله عليه وسلم يَسْتَمِعُ قِرَاءَةَ رَجُلٍ فِي الْمَسْجِدِ . فَقَالَ " رَحِمَهُ اللَّهُ لَقَدْ أَذْكَرَنِي آيَةً كُنْتُ أُنْسِيتُهَا " .
May Allah have mercy upon him; be reminded me of the verse which I had been made to forget.