Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "How bad it is for a man or a woman to say: 'I have forgotten such-and-such a surah or such-and-such a verse.' Rather, (the truth is that) he has been made to forget it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10398