Hadith 2188
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”.)