Praise be to Allah, and may peace and blessings be upon the Messenger of Allah.
A Muslim may not deliberately add anything to the Quran, omit anything from it, or alter its wording, even because of forgetfulness or fear of failing. Whoever deliberately changes a Quranic expression while knowing that it is not part of the Quran has committed a very serious act.
If a Muslim makes a mistake while reciting the Quran or forgets part of it, they should try to correct the mistake and recall what they have forgotten. If they are unable to do so, they may skip the passage they have forgotten and continue from what follows, or move to another surah. They should not invent a word and place it in the position of the Quranic wording.
This ruling applies not only during prayer but also to reading and writing the Quran outside prayer.
The Quran is the word of Allah تعالى. It has been transmitted through mass transmission, and its wording is divinely fixed. Therefore, it is impermissible to deliberately alter any of its wording, whether or not the alteration changes the meaning.
Scholars have emphasized the seriousness of deliberately omitting a letter from the Quran, replacing one letter with another, or adding a letter or word while knowing that it is not part of the Quran.
Al-Qadi ‘Iyad رحمه الله stated that Muslims unanimously agree that the recited Quran found throughout the world and written in the copies of the Quran in the hands of Muslims—from the beginning of Al-Fatihah to the end of An-Nas—is the word of Allah and His revelation sent down to His Prophet Muhammad ﷺ, and that whoever deliberately omits a letter from it, replaces it with another letter, or deliberately adds something that is not part of the agreed-upon Quran is a disbeliever.
Scholars have likewise stated in the Fiqh Encyclopedia that deliberately committing a grammatical or recitational error in the Quran is prohibited, whether it changes the meaning or not, because the wording of the Quran has been transmitted through mass transmission and must not be deliberately altered by changing its letters or replacing one letter with another.
Accordingly, a student may not write a word or letter during an examination while knowing that it is not part of the Quran, nor may they replace a forgotten Quranic word with another word merely because they want a higher grade or fear failing.
Forgetfulness is an excuse, but deliberately attributing words to the Quran while knowing that they are not part of it is not an excuse.
If a student forgets a word or letter, they should try to remember it. If they cannot, they should leave the passage incomplete rather than insert something of their own. They may explain that they cannot complete the verse because they have forgotten the wording and do not want to state something they are not certain about.
The statements transmitted from scholars concerning someone who deliberately omits part of the Quran or adds something that is not from it, while knowing that it is not Quran, are extremely serious. Some scholars explicitly judged this specific act to constitute disbelief because it involves deliberately altering the word of Allah and attributing something that is not from Him to the Quran.
However, it is essential to distinguish between an ordinary mistake or forgetfulness and deliberate alteration while knowing the truth. A student who forgets a verse or word and makes an unintended mistake does not receive the ruling of someone who deliberately alters the Quran.
As for someone who deliberately inserts wording while knowing that it is not part of the Quran, this is the action that scholars have strongly warned against. It is not permissible to do so on the pretext of an examination or fear of failing.
It is not permissible to deliberately add a letter or word to the Quran, omit one, or replace one with another, whether during prayer or outside it. Whoever forgets part of the Quran should stop, skip the forgotten passage, or otherwise avoid inserting wording that they know is not from the Quran.
Scholars have stated that deliberately altering the Quran in this manner, while knowing that one is altering the word of Allah, is a matter that may constitute disbelief.
And Allah knows best.