Sahih Muslim Book 4, Hadith Number 1727.

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.

Abu Musa al-Ash’ari reported Allah’s Apostle (may peace be upon him) as saying: Keep refreshing your knowledge of the Qur’an, for I swear by Him in Whose Hand is the life of Muhammad that it is more liable to escape than camels which are hobbled.