Sahih Muslim 791
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
Abu Musa al-Ash'ari reported Allah's Apostle (ﷺ) as saying:
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 .
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.