Sahih Muslim Book - 6, Hadith - 1842

حَدَّثَنَا ابْنُ نُمَيْرٍ حَدَّثَنَا أَبِى 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 نُسِّىَ ».

It was narrated that Shaqiq said: "'Abdullah said: 'Keep refreshing your knowledge of these Masahif - or he said, 'the Qur'an - for it is more inclined to escape from the hearts of men than camels from their ropes. And the Messenger of Allah (s.a.w) said: 'None of you should say: I have forgotten such-and-such a verse,' rather he has been caused to forget.'"