Sahih Muslim Book 4, Hadith Number 1722.

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 b. ‘Umar reported Allah’s Messenger (may peace be upon him) as saying: The example of a man who has memorised the Qur’an is like that of a hobbled camel. If he remained vigilant, he would be able to retain it (with him), and if he loosened the hobbled camel it would escape.