Sahih al-Bukhari 5039
Sahih al-Bukhari · Book 66: Virtues of the Qur'an · Forgetting the Qur'an. And can one say: “I forgot such and such a Verse?
Narrated `Abdullah:
حَدَّثَنَا أَبُو نُعَيْمٍ ، حَدَّثَنَا سُفْيَانُ ، عَنْ مَنْصُورٍ ، عَنْ أَبِي وَائِلٍ ، عَنْ عَبْدِ اللَّهِ ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم " مَا لأَحَدِهِمْ يَقُولُ نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ. بَلْ هُوَ نُسِّيَ " .
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."