كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(33)
Chapter: 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
(33)
باب الأَمْرِ بِتَعَهُّدِ الْقُرْآنِ وَكَرَاهَةِ قَوْلِ نَسِيتُ آيَةَ كَذَا. وَجَوَازِ قَوْلِ أُنْسِيتُهَا
Sahih Muslim 788 b

'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.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 788b
In-book reference : Book 6, Hadith 266
USC-MSA web (English) reference : Book 4, Hadith 1721
  (deprecated numbering scheme)