Sahih Muslim 790 b
'Abdullah is reported to have said:
Keep refreshing your knowledge of the sacred books (or always renew your knowledge of these sacred books) and sometimes he would mention the Qur'an for it is more apt to escape from men's minds than animals which are hobbled, and the Messenger of Allah (ﷺ) said: None of you should say: I forgot such and such a verse, but he has been made to forget.
حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي وَأَبُو مُعَاوِيَةَ ح وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى، - وَاللَّفْظُ لَهُ - قَالَ أَخْبَرَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ شَقِيقٍ، قَالَ قَالَ عَبْدُ اللَّهِ تَعَاهَدُوا هَذِهِ الْمَصَاحِفَ - وَرُبَّمَا قَالَ الْقُرْآنَ - فَلَهُوَ أَشَدُّ تَفَصِّيًا مِنْ صُدُورِ الرِّجَالِ مِنَ النَّعَمِ مِنْ عُقُلِهِ ‏.‏ قَالَ وَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ لاَ يَقُلْ أَحَدُكُمْ نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ بَلْ هُوَ نُسِّيَ ‏"‏ ‏.‏