Sahih al-Bukhari 5032
Sahih al-Bukhari · Book 66: Virtues of the Qur'an · The learning of the Qur'an by heart and the reciting of it repeatedly
Narrated `Abdullah:
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 " .
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."