Sahih Bukhari
Virtues of the Quran
Volume: 006 / Book Number: 061 / Hadith Number: 549
Narrated Ibn Umar:
Allah's Apostle said, "The example of the person who knows the Qur'an by heart is like the owner of tied camels. If he keeps them tied, he will control them, but if he releases them, they will run away."