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 guarding the Qur'an, for it is wild by nature; it escapes from the breasts of men even faster than a camel escapes from its tether. And let none of you say: I have forgotten such-and-such a verse; rather, let him say: I was made to forget it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10193