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." (Using translation from Bukhārī 5032)   

عَنِ النَّبِيِّ ﷺ قَالَ بِئْسَمَا لَأَحَدِهِمْ أَنْ يَقُولَ «نَسِيتُ آيَةَ كَيْتَ اسْتَذْكِرُوا الْقُرْآنَ فَإِنَّهُ أَسْرَعُ تَفَصِّيًا مِنْ صُدُورِ الرِّجَالِ مِنَ النَّعَمِ مِنْ عُقُلِهِ» وَقَفَهُ جَرِيرٌ  

What a wretched person is he amongst them who says: I have forgotten such and such a verse. (He should instead of using this expression say): I have been made to forget it. Try to remember the Qur'an for it is more apt to escape from men's minds than a hobbled camel. (Using translation from Muslim 790a)   

اسْتَذْكِرُوا الْقُرْآنَ فَلَهُوَ أَشَدُّ تَفَصِّيًا مِنْ صُدُورِ الرِّجَالِ مِنَ النَّعَمِ مِنْ عُقُلِهِ وَلَا يَقُولَنَّ أَحَدُكُمْ نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ قَالَ قَالَ رَسُولُ اللهِ ﷺ «بَلْ هُوَ نُسِّيَ»