Riyad as-Salihin

Ibn 'Umar (May Allah be pleased with them) reported: The Messenger of Allah (ﷺ) said: "The parable of one who knows the Qur'an by heart is as the parable of an owner of hobbled camel. If he remains vigilant, he will retain it; and if he neglects it, it will go away."

[Al-Bukhari and Muslim].