Sayyiduna Abdullah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "It is a bad thing for any of you to say: I have forgotten such-and-such a verse; rather (let him say): it was made to be forgotten. Keep refreshing your memory of the Qur'an, for it surely escapes from people's breasts more quickly than animals escape from their tethers."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4053