Sahih Muslim Book 4, Hadith Number 1726.

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.

Ibn Mas’ud reported Allah’s Messenger (may peace be upon him) as saying: Wretched is the man who says: I forgot such and such a sura, or I forget such and such a verse, but he has been made to forget.