Sahih Muslim
Sahih Muslim 788 b
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
Arabic hadith text
وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا عَبْدَةُ، وَأَبُو مُعَاوِيَةَ عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ كَانَ النَّبِيُّ صلى الله عليه وسلميَسْتَمِعُ قِرَاءَةَ رَجُلٍ فِي الْمَسْجِدِ . فَقَالَ " رَحِمَهُ اللَّهُ لَقَدْ أَذْكَرَنِي آيَةً كُنْتُ أُنْسِيتُهَا "
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.