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: "How bad it is for one of you to say: 'I have forgotten such-and-such a verse.' Rather, (the truth is that) he has been made to forget it. Keep guarding the Qur'an, for it is wild (prone to bolt), and it escapes from the breasts of men faster than camels break loose from their tethers."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10377