Sayyiduna Ibn Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "It is extremely bad for any one of you to say, 'I forgot such-and-such verse,' rather he should say, 'Such-and-such verse was made to be forgotten by me.' Safeguard this Qur'an, for it slips away from people's hearts faster than a camel escapes from its rope."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4176