Forgetting the Quran out of Negligence is a Sin
8-9-2018 | IslamWeb
Question:
I memorised some Surahs and a few scattered sentences from the Quran but forgot them due to negligence. Recently I found that some scholars, like Ibn Taymiyyah and Abu Al-'Aaliyah consider this a grave major sin or a minor sin. I know that some modern scholars say the hadiths a weak but still it could be a sin. my question is: am I OBLIGED to re-memorise the forgotten bits and keep them memorised from now on? I know it is better to do that but I want to know if it is a sin if I don't.If the answer is yes then I have some major problems. I barely know the Arabic alphabet and I can't read Quran so I would have to listen to the recitation to try and imitate it. This is very difficult and I would make many grave errors that probably change the meaning every time I recited. I can't actually pronounce some letters correctly. And my memory is poor sometimes so it would be very difficult and time consuming. Also I can't remember which exact phrases I memorised by heart. So would I have to memorise extra Quran around the phrases I think I memorised to make sure I didn't miss out some bits?
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
There is no doubt that forgetting the Quran after its memorization, neglecting it and not attaching any importance to it is considered to be carelessness regarding this great blessing and being ungrateful to it. This is undoubtedly a dispraised matter and some scholars even regarded it as a major sin as you mentioned, among whom was Shaykh Ibn Taymiyyah 
.
Therefore, since the case is as you mentioned, that you forgot the Quran due to negligence, then you should take the initiative to sincerely repent to Allah Almighty. You should also hasten to review it so that you will retain it as stated by Ar-Ramli in his Fataawa:
“If one forgets it [the Quran] –while he is an adult –out of negligence and laziness, then this is a grave major sin; he must memorize it (again) if he is able to do so, in order to get rid of the sin.” [End of quote]
We think that the matter in this regard is easy, because fulfilling religious commands and obligations depends on ability; Allah Says (what means): {So fear Allah as much as you are able to.} [Quran 64:16]
Therefore, you should endeavor to know the exact verses that you forgot and if you cannot remember which ones they are, then you are not obliged to memorize other verses in their place.
Memorizing the Quran in general is recommended in principle, so you should try to memorize as much of it as possible. You should look for a Shaykh who would teach you how to recite it correctly. If you cannot find one, then you can benefit from audio clips, especially the Mahmood Khaleel Al-Husari's recitation of the Quran (the teaching version, which you can download from the internet), as well as others.
For more benefit, please refer to Fataawa 372070 and 273346.
Allah knows best.