Ibn `Umar (May Allah be pleased with them) reported: The Messenger of Allah (PBUH) 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].

Commentary: This Hadith repeats the subject mentioned above. The person who memorizes the entire Qur'an or a portion of it, if he recites it regularly, he will retain it; otherwise, it will start slipping from his memory.