Sahih Muslim Book 4, Hadith Number 1723.

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.

This Hadith has been narrated by Ibn ‘Umar from the Apostle of Allah (may peace be upon him), but in the Hadith transmitted by Musa b. ‘Uqba, this addition is made: “When one who had committed the Qur’an to memory(or who is familiar with it) gets up (for night prayer) and recites it night and day, it remains fresh in his mind, but if he does not get up (for prayer and thus does not recite it) he forgets it.”