Is it a sin to forget Quran or part of Quran after remembering it? Is it included in major sins?

Is it a sin to forget Quran or part of Quran after remembering it?  Is it included in major sins?

Assalaam alaikum,
When we memorize the Qur'an, we do it to please Allah, to learn more of His book, and to earn the rewards promised those that memorize the Qur'an on the Day of Judgement.  We then need to keep a regular program of review to make sure what we memorized stays memorized.
The Prophet, peace and blessings of Allah upon him explained the need to keep reading and reviewing the Qur'an.
In a  hadeeth narrated by al-Bukhari, Ibn ‘Umar (may Allah be pleased with him) said  that the Messenger of Allah  (peace and blessings of Allah upon him) said: “The likeness of the one who memorizes the Qur’an is that of the owner of a hobbled camel.  If he tends to it regularly, he will keep it, but if he lets it go, he will lose it.” al-Bukhari.

If one does his best at review and always keeping a reasonable schedule of review trying to keep up the memorization, then insha' Allah, they will keep what they memorized and receive the reward promised by Allah.
 It was narrated that ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) said: The Messenger of Allah (peace and blessings of Allah be upon him) said: It will be said to the companion of the Qur'an [on the day of Judgement]: Recite and rise; recite as you used to recite in the world for your level will be at the last verse you recite.” Narrated by Abu Dawood  and graded as saheeh by al-Albaani in Saheeh Abi Dawood.

If however one doesn't try to review and lets it laspe with no effort to keep it up, then they have not done the work required. 

As to the question of whether to forget it is a major sin, some quote a hadeeth, “The sins of my ummah were shown to me and there is no sin greater than that of a man who was given or  was helped to memorize a surah or an aayah of the Qur’aan and then he forgot it.” but this is a weak hadeeth and was graded weal by Imam al-Bukhari and At-Tirmidhi, and related as such by Albani.  The hadeeth being weak doesn't mean we should memorize and feel at ease about forgetting.  Our righteous predecessors were always conscious of memorizing and reviewing and feared forgetting what they had memorized and considered forgetting to be a shortcoming.