«بئسما لأحدكم أن يقول: نسيت آية كيت وكيت بل هو نُسِّيَ»
The Prophet (peace and blessings be upon him) said:
"How bad it is for one of you to say, 'I have forgotten such-and-such a verse'; rather, he has in fact been made to forget it (so he should say that he was made to forget it)."
Hadith Referenceصحیح الجامع الصغیر / حرف الباء / 2847