Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1723
This hadith has been narrated by Ibn 'Umar from the Apostle of Allah (may peace be upon him), but in the hadith transmited 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."