Sayyiduna Ibn Mas'ud (may Allah be pleased with him) says: Safeguard this Qur'an, for it slips away from people's hearts faster than a camel escapes from its rope. And he said that the Messenger of Allah (peace and blessings be upon him) said: "It is a very bad thing for a person to say, 'I forgot such-and-such verse,' rather he should say, 'It was made to be forgotten by me.'"
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4020