إِنَّا نَحْنُ نَزَّلْنَا ٱلذِّكْرَ وَإِنَّا لَهُۥ لَحَٰفِظُونَ ﴿٩﴾
Indeed, it is We who sent down the Qur'an and indeed, We will be its guardian.
Short answer
Muslims believe the Quran has been preserved word for word. God promised to protect it, and it has been preserved in two ways: in writing and, even more, in the memory of millions of people who learn it by heart.
The fuller answer
From the time of the Prophet ﷺ, companions memorised the Quran and checked each other's recitation. Under the third caliph, 'Uthman, a standard copy was written and copies were sent to the main cities. The oldest surviving manuscripts, such as the Birmingham fragments and the Topkapi and Samarkand copies, match today's text. Scholars who have studied them, Muslim and non-Muslim, note that the text is remarkably stable.
Today, very many Muslim children and adults memorise the whole Quran every year, and a Muslim in China, Nigeria and Indonesia recites the same words. Muslims cite this unbroken chain of memorisation and recitation as the strongest proof that nothing has been added or removed. Variant readings (qira'at) exist, but they are authentic and traced to the Prophet ﷺ.
From the Quran
Please note
This answer follows the mainstream Sunni understanding and is general learning, not a fatwa. Where Muslims differ, the answer says so. Have a question of your own? Write to us.