Sahih al-Bukhari 5032
Narrated `Abdullah:
The Prophet (ﷺ) said, "It is a bad thing that some of you say, 'I have forgotten such-and-such verse of the 
Qur'an,' for indeed, he has been caused (by Allah) to forget it. So you must keep on reciting the Qur'an 
because it escapes from the hearts of men faster than camel do."
حَدَّثَنَا مُحَمَّدُ بْنُ عَرْعَرَةَ، حَدَّثَنَا شُعْبَةُ، عَنْ مَنْصُورٍ، عَنْ أَبِي وَائِلٍ، عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم ‏
"‏ بِئْسَ مَا لأَحَدِهِمْ أَنْ يَقُولَ نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ بَلْ نُسِّيَ، وَاسْتَذْكِرُوا الْقُرْآنَ فَإِنَّهُ أَشَدُّ تَفَصِّيًا مِنْ صُدُورِ الرِّجَالِ مِنَ النَّعَمِ ‏"‏‏.‏