Sahih al-Bukhari, Volume 6, Book 61, Hadith 559 (USC-MSA) ⓘ
Narrated `Abdullah: The Prophet 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."
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 " .
also referenced under: Book 66, Hadith 63 (In-Book) & Hadith 5039 (DarusSalam)