It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Keep striving to remember the Qur'an, for it escapes from the hearts of men faster than livestock break loose from their tethers. And how evil it is for one of you to say, 'I forgot such-and-such a verse'; rather (the truth is that) he was made to forget it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10411