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