Language:
كتاب فضائل القرآن
66
Virtues of the Qur'an
(23)
Chapter: The learning of the Qur'an by heart and the reciting of it repeatedly
(23)
باب اسْتِذْكَارِ الْقُرْآنِ وَتَعَاهُدِهِ

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 5032
In-book reference : Book 66, Hadith 54
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 550
  (deprecated numbering scheme)