Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1722
'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.