The Book of Prayers (Kitab Al-Salat)
Muslim :: Book 4 : Hadith 1724
'Abdullah reported Allah's Messenger (may peace be upon him) as saying: What a wretched person is he amongst them who says: I have forgotten such and such a verse. (He should instead of using this expression say): I have been made to forget it. Try to remember the Qur'an for it is more apt to escape from men's minds than a hobbled camel.