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

'A'isha reported that the Messenger of Allah (ﷺ) heard a person reciting the Qur'an at night. Upon this he said:

May Allah show mercy to him; he has reminded me of such and such a verse which I had missed in such and such a surah.
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ هِشَامٍ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم سَمِعَ رَجُلاً يَقْرَأُ مِنَ اللَّيْلِ فَقَالَ ‏ "‏ يَرْحَمُهُ اللَّهُ لَقَدْ أَذْكَرَنِي كَذَا وَكَذَا آيَةً كُنْتُ أَسْقَطْتُهَا مِنْ سُورَةِ كَذَا وَكَذَا ‏"‏ ‏.‏
Reference : Sahih Muslim 788 a
In-book reference : Book 6, Hadith 265
USC-MSA web (English) reference : Book 4, Hadith 1720
  (deprecated numbering scheme)