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].
App reference: Book 9, Hadith 13
Arabic/English book reference: Book 9, Hadith 1003