The Prophet ﷺ said, "Why does anyone of the people say, 'I have forgotten such-and-such Verses (of the Qur'an)?' He, in fact, is caused (by Allah) to forget." (Using translation from Bukhārī 5039)   

قَالَ رَسُولُ اللهِ ﷺ بِئْسَمَا لَأَحَدِهِمْ أَنْ يَقُولَ نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ بَلْ هُوَ نُسِّيَ  

[Machine] "That the Messenger of Allah ﷺ said: 'Verily the example of the Qur'an is like that of a tied camel. If its owner takes care of it, it remains with him, and if he neglects it, it goes away. Similarly, when a person establishes a relationship with the Qur'an and recites it day and night, he remembers it, but when he does not recite it, he forgets it.'"  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «إِنَّمَا مَثَلُ الْقُرْآنِ كَمَثَلِ الْإِبِلِ الْمُعَقَّلَةِ إِذَا عَاهَدَهَا صَاحِبُهَا عَلَى عُقُلِهَا أَمْسَكَهَا وَإِذَا أَغْفَلَهَا ذَهَبَتْ إِذَا قَامَ صَاحِبُ الْقُرْآنِ فَقَرَأَهُ بِاللَّيْلِ وَالنَّهَارِ ذَكَرَهُ وَإِذَا لَمْ يَقْرَأْهُ نَسِيَهُ»