Narrated by Abdullah ibn Umar(Radhiallaho anho):
Allah’s Messenger (sallAllaahu ‘alaihi wasallam) said: 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.

Sahih Muslim Number 1722