Sahih Muslim Book 4, Hadith Number 1724.

Chapter : Concerning dozing off in prayer, or faltering of one’s tongue in the recitation of the Qur’an, or in mentioning of Allah, one should sleep, or stop it till one becomes lively.

‘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.