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