‘Abdullāh ibn Mas‘ūd (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "How terrible it is for someone to say: ‘I have forgotten such-and-such verse of the Qur’an', for indeed, he has been caused to forget it. So, you must keep on memorizing the Qur'an for it is more likely to escape from the hearts of men faster than camels.”
In this Hadīth, the Prophet (may Allah's peace and blessings be upon him) criticizes whoever says that he has forgotten such-and-such verse of the Qur'an, because it gives the impression that he takes the issue of neglecting the Qur'an and forgetting it indifferently. Actually, the Prophet (may Allah's peace and blessings be upon him) added “for indeed, he has been caused to forget it”, meaning: he was punished by forgetfulness for neglecting the regular recitation and memorization of the Qur'an. After that, the Prophet (may Allah's peace and blessings be upon him) commanded Muslims to keep refreshing their knowledge of the Qur’an by regularly reciting, memorizing, and studying it since it is more likely to escape from the hearts of men faster than camels. The camel was mentioned in particular here because it is the readiest of domestic animals to run away, and if it does run away, recapturing it is very difficult.