Mishkat al-Masabih
The Excellent Qualities of the Qur'an — Hadith #2091
وَعَنِ ابْنِ مَسْعُودٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم:
" بئس مالأحدهم أَنْ يَقُولَ: نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ بَلْ نُسِّيَ وَاسْتَذْكِرُوا الْقُرْآنَ فَإِنَّهُ أَشَدُّ تَفَصِّيًا مِنْ صُدُورِ الرِّجَالِ مِنَ النَّعَمِ ". مُتَّفَقٌ عَلَيْهِ. وَزَادَ مُسلم: «بعقلها»
Ibn Mas’ūd reported God’s messenger as saying, “It is wrong for one to say that he has forgotten such and such a verse, for he has been made to forget. Study the Qur’ān, for it is more apt to escape from men’s minds than animals.”
(Bukhārī and Muslim, Muslim adding “which are tethered”.)