Narrated Ibn `Umar
Allah's Messenger (ﷺ) said, "The example of the person who knows the Qur'an by heart is like the owner of tied camels. If he keeps them tied, he will control them, but if he releases them, they will run away."
Narrated `Abdullah
The Prophet (ﷺ) said, "It is a bad thing that some of you say, 'I have forgotten such-and-such verse of the Qur'an,' for indeed, he has been caused (by Allah) to forget it. So you must keep on reciting the Qur'an because it escapes from the hearts of men faster than camel do."
Narrated Abu Musa
The Prophet (ﷺ) said, "Keep on reciting the Qur'an, for, by Him in Whose Hand my life is, Qur'an runs away (is forgotten) faster than camels that are released from their tying ropes."