From Sahih Muslim (Hadith 1839):
Abdullah b. 'Umar reported Allah's Messenger (ﷺ) 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
Reference: Muslim 1839