Assalamualaikum, I’m a tahfiz school graduate who has memorized all 30 juzuk of al-Quran. I always worry if I forgot what I have memorized before. My question is about what is the ruling if I forgot the Quran that I’ve memorized?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions and all those who follow his footsteps until the Last Day.
Memorizing al-Quran is an honoured and commendable act of which whoever does it will be greatly rewarded for he is protecting the kalamullah. Furthermore, memorizing the whole Quran is fardhu kifayah and sunnah ‘ain, which sunnah for every Muslim. 
However, although memorizing al-Quran is an honourable act, what scares most Quran memorizers is to maintain their memorization. This could demotivate a person from memorizing the Quran for he is afraid that he’ll forget and is said to have sinned. In answering this question, we’ll cite several opinions:
In this issue, we’d like to remind all of us of a hadith from Abdullah RA:
بِئْسَمَا لأَحَدِهِمْ يَقُولُ نَسِيتُ آيَةَ كَيْتَ وَكَيْتَ بَلْ هُوَ نُسِّيَ اسْتَذْكِرُوا الْقُرْآنَ فَلَهُوَ أَشَدُّ تَفَصِّيًا مِنْ صُدُورِ الرِّجَالِ مِنَ النَّعَمِ بِعُقُلِهَا
“What a wretched person is he amongst them who says: I have forgotten such and such a verse. (He should instead of using this expression say): I have been made to forget it. Try to remember the Qur’an for it is more apt to escape from men’s minds than a hobbled camel.” 
Khatib Syarbini said: According to this hadith, it is sunnah to use the phrase made to forget the Quran rather than using the word, forgotten or forget. 
From Sa’ad bin Ubadah RA, the Messenger PBUH said:
مَا مِنْ امْرِئٍ يَقْرَأُ القُرآنَ ثُمَّ يَنْسَاهُ إِلَّا لَقِيَ اللهَ عَزَّ وَجَلَّ أَجْذَمْ
“No man recites the Qur’an, then forgets it, but will meet Allah on the Day of Judgment in a maimed condition (or empty-handed, or with no excuse).” 
Al-Munziri said that in it there is a narrator named Yazid bin Abi Ziyad of whom his hadiths are not used as hujjah (evidence). 
Al-Khattabi cited the words of Abu Ubaid who said: “Ajzam” in the above hadith means maimed hand, while Ibn Qutaibah said it means dwarfed. 
Some scholars used the above hadith as evidence in the issue of forgetting the memorization of al-Quran. It is stated in the explanation of the word “يَقْرَأُ” above includes the meaning of reciting by reading, reciting through memorization and reciting the meaning of al-Quran then neglect its recitation purposely or because he forgets. However, the above hadith is evaluated as dhaif (weak).
From Anas bin Malik RA, Rasullullah PBUH said:
عُرِضَتْ عَلَيَّ ذُنُوْبُ أُمَّتِي فَلَمْ أَرَ ذَنْباً أَعْظَمُ مِنْ سُوْرَةٍ مِنَ القُرآنِ أُوْتِيَهَا رَجُلٌ ثُمَّ نَسِيَهَا
“The sins of my people were placed before me, and I saw no sin greater than that a man should memorise a sura or verse of the Qur’an, then forget it.” 
This hadith is evaluated as Gharib by Imam Tirmizi himself, while al-Munziri stated it is dhaif for there is a narrator named Abd Majis bin Abd Aziz. 
Furthermore, Ali bin al-Madini also stated that Al-Mutthalib bin Abdillah who narrated this hadith from Anas bin Malik actually has never heard any hadith from Anas. 
Salaf scholars have differing opinions in the chapter of forgetting the memorization of the verses of al-Quran.
Some of them ruled it as a major sin. Among them is jumhur of the scholars, some Syafie scholars such as Syeikh al-Rauyani, al-Qurtubi and others who state, negligence of the reciter as the reason he forgets the verse, this forgetfulness shows that he didn’t give his ample attention to the Quran.
- Ibn hjar states: It is said in a narration from Abu Ubaid through the route of al-Dahhak through Mauquf:
مَا مِنْ أَحَدٍ تَعَلَّمَ القُرآنَ ثُمَّ نَسِيَهُ إلَّا بِذَنْبٍ أَحْدَثَهُ لِأَنَّ اللهَ يَقُوْلُ: وَمَا أَصَابَكُمْ مِنْ مُصِيْبَةٍ فَبِمَا كَسَبَتْ أَيْدِيْكُمْ
There is none of you who learns al-Quran then forgets it except you have sinned for the statement made by Allah SWT: “There is no calamity which befalls you except it is due to your own hands.”
“They added: There is no greater calamity than forgetting the Quran.” Another continuation of the above hadith but of mursal status according to al-Hafiz Ibn Hajar al-Asqalani; Meaning: “… There is no sin greater that I see, except the sin of a person who memorized the Quran but then he left it.” 
- According to Syeikh Zakariyya al-Ansari, it is haram to forget the memorization of al-Quran based on the above-mentioned hadiths. 
- While according to Imam al-Ramli, it is haram if it is due to a person’s inattentiveness or negligence and is one of the major sins. 
- Whereas Imam al-Syaukani said, forgetting the memorization of al-Quran is the greatest sin among the minor sins if it isn’t the person’s intention to belittle the honour of al-Quran. 
- Ibn Husamuddin al-Rahmani said, the threat to those who forget the memorization of al-Quran is because al-Quran is the centre of the teachings of Islam and shariah. Forgetting it means damaging the shariah itself. Memorization of al-Quran is one of Allah SWT’s honoured blessings hence, forgetting it means a person has been ungrateful for the blessings he has been given. 
- Ibn Rusyd also cited an ijma’ where if a person forgot the memorization of al-Quran because he is busy studying other obligatory and sunnah knowledge, then he isn’t sinful. 
- Abu Syammah, the teacher of Imam al-Nawawi interprets the word نسيان to mean leaving it and not practise it and not in terms of just its memorization. 
While if a person forgets due to the natural character of a human, then it isn’t sinful for the Prophet PBUH himself once forgotten some of the verses of al-Quran. From ‘Aisyah R.Anha:
سَمِعَ رَسُولُ اللَّهِ صلى الله عليه وسلم رَجُلاً يَقْرَأُ فِي سُورَةٍ بِاللَّيْلِ فَقَالَ “ يَرْحَمُهُ اللَّهُ لَقَدْ أَذْكَرَنِي كَذَا وَكَذَا آيَةً كُنْتُ أُنْسِيتُهَا مِنْ سُورَةِ كَذَا وَكَذَا
“Allah’s Messenger (ﷺ) heard a man reciting the Qur’an at night, and said, “May Allah bestow His Mercy on him, as he has reminded me of such-and-such Verses of such-and-such Suras, which I was caused to forget.” 
Thus, the Prophet PBUH advised and gave tips on how to maintain one’s memorization of al-Quran through his words:
وَإِذَا قَامَ صَاحِبُ الْقُرْآنِ فَقَرَأَهُ بِاللَّيْلِ وَالنَّهَارِ ذَكَرَهُ وَإِذَا لَمْ يَقُمْ بِهِ نَسِيَهُ
” When one who had committed the Qur’an to memory (or who is familiar with it) gets up (for night prayer) and recites it night and day, it remains fresh in his mind, but if he does not get up (for prayer and thus does not recite it) he forgets it.” 
However, some salaf scholars explained that the meaning of made to forget here is forgetting to practise the verse. The reason is if the warning of forgetting here is subject to forgetting the memorization of al-Quran then surely a person will say, then it will be better to not memorize any verse of the Quran at all. Hence, this is contradictory to the objective of shariah.
Whatever the interpretation, strengths and weaknesses of the narrators of the hadiths, it is obligatory to maintain the memorization of al-Quran to retain its high status according to Allah SWT and it is unreasonable to learn knowledge from al-Quran and understood it but then just forgets it. Thus, we must maintain the memorization and knowledge from al-Quran to be recited and practised.
We are of the opinion in the issue of forgetting the memorization of al-Quran that has been discussed thoroughly by fiqh scholars as to the following:
- Madhhab Hanafi. Al-Khadimi said: Forgetting al-Quran after studying it is included as a sin for his negligence to review and repeat it. 
- Madhhab Maliki. Ibn Abd al-Bar said: Hadith regarding sahih al-Quran to learn al-Quran and always recite it. Moreover, it also gives a stern warning from forgetting it. 
- Madhhab Syafie. Syeikh al-Khatib al-Syarbini said: Forgetting al-Quran is included as a major sin. 
- Madhhab Hanbali. Syeikh Musthafa al-Rahibani said: It is prohibited to delay the khatam of al-Quran for more than 40 days for concern of forgetting it. 
According to our analysis, the ruling of forgetting the memorization of al-Quran is divided into two:
- If one is negligent in reviewing and repeating it because of inattentiveness and worldly affairs, then it is sinful as stated by the fatwa of most scholars.
- If he has tried to remember and memorize as well as reviewing and repeating it, but he still forgets, then it is not sinful.
This is in line with the opinion of Ibn Rusyd al-Maliki who cited through ijmak: Whoever forgets al-Quran because he is busy earning other obligatory or sunnah knowledge, then it is not a sin.