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

Abu Musa al-Ash'ari reported Allah's Apostle (ﷺ) as saying:

Keep refreshing your knowledge of the Qur'an, for I swear by Him in Whose Hand is the life of Mahammad that it is more liable to escape than camels which are hobbled.
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ بَرَّادٍ الأَشْعَرِيُّ، وَأَبُو كُرَيْبٍ قَالاَ حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ بُرَيْدٍ، عَنْ أَبِي بُرْدَةَ، عَنْ أَبِي مُوسَى، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ تَعَاهَدُوا هَذَا الْقُرْآنَ فَوَالَّذِي نَفْسُ مُحَمَّدٍ بِيَدِهِ لَهُوَ أَشَدُّ تَفَلُّتًا مِنَ الإِبِلِ فِي عُقُلِهَا ‏"‏ ‏.‏ وَلَفْظُ الْحَدِيثِ لاِبْنِ بَرَّادٍ.
Reference : Sahih Muslim 791
In-book reference : Book 6, Hadith 272
USC-MSA web (English) reference : Book 4, Hadith 1727
  (deprecated numbering scheme)