Short answer
Deliberately burning, tearing or throwing the Quran into filth to insult it is one of the gravest sins, and a Muslim who does it out of contempt leaves Islam; the four schools agree. Retiring a worn-out copy with respect is different: the Hanafis prefer burial, and burning it completely with respect is not desecration.
Insulting the mushaf on purpose, by burning it, trampling it or throwing it in filth with contempt, is kufr (disbelief) for a Muslim. An unusable copy is retired with respect: the Hanafi preference is to wrap it and bury it, and many later Hanafi muftis allow burning it completely with respect where burial is not possible.
The four schools agree on this.
Explanation
The Quran is the speech of Allah, and honouring the written copy, the mushaf, is part of honouring Allah. The Quran says that whoever honours the symbols of Allah does so from the piety of hearts (22:32). When some people mocked Allah and His verses, the Quran said: make no excuse, you have disbelieved after your belief (9:65 to 66). For this reason the four schools agree that a Muslim who deliberately burns, tears, steps on or throws the mushaf into filth to show contempt has committed disbelief.
Intention makes the difference. Burning an old, damaged copy that can no longer be read, with respect and with the aim of protecting it from misuse, is not desecration. The Companions burned the extra copies when Uthman sent out the standard mushaf. Even so, the Hanafi scholars prefer to wrap a worn copy and bury it, as a Muslim's body is buried. See how to dispose of an old Quran and why Uthman burned copies of the Quran.
When others do it. Muslims feel deep pain when someone burns the Quran in public. The Quran prepared the believers for this: you will hear much abuse from others, and if you are patient and fear Allah, that is a matter of firm resolve (3:186). The proper response is lawful: speaking out, using the law and the media, explaining what the Quran teaches, and making dua. Taking revenge on individuals is not allowed, and no one may take the law into their own hands. Insulting what others hold sacred in reply is also forbidden (6:108).
The Prophet ﷺ told the Companions not to travel with the Quran into enemy land, fearing that it would be mistreated. Scholars take from this that Muslims should protect copies of the Quran from places where they are likely to be abused.
The evidence
Quran, Surah Al-Hajj 22:32
ذَٰلِكَ وَمَن يُعَظِّمْ شَعَٰٓئِرَ ٱللَّهِ فَإِنَّهَا مِن تَقْوَى ٱلْقُلُوبِ ﴿٣٢﴾
That [is so]. And whoever honors the symbols of Allah - indeed, it is from the piety of hearts.
Honouring what Allah has made sacred, including His Book, is a sign of piety.
Quran, Surah At-Tawba 9:65-66
وَلَئِن سَأَلْتَهُمْ لَيَقُولُنَّ إِنَّمَا كُنَّا نَخُوضُ وَنَلْعَبُ ۚ قُلْ أَبِٱللَّهِ وَءَايَٰتِهِۦ وَرَسُولِهِۦ كُنتُمْ تَسْتَهْزِءُونَ ﴿٦٥﴾ لَا تَعْتَذِرُوا۟ قَدْ كَفَرْتُم بَعْدَ إِيمَٰنِكُمْ ۚ إِن نَّعْفُ عَن طَآئِفَةٍۢ مِّنكُمْ نُعَذِّبْ طَآئِفَةًۢ بِأَنَّهُمْ كَانُوا۟ مُجْرِمِينَ ﴿٦٦﴾
And if you ask them, they will surely say, "We were only conversing and playing." Say, "Is it Allah and His verses and His Messenger that you were mocking?" Make no excuse; you have disbelieved after your belief. If We pardon one faction of you - We will punish another faction because they were criminals.
Mocking the verses of Allah is called disbelief after belief.
Quran, Surah Aal-i-Imraan 3:186
۞ لَتُبْلَوُنَّ فِىٓ أَمْوَٰلِكُمْ وَأَنفُسِكُمْ وَلَتَسْمَعُنَّ مِنَ ٱلَّذِينَ أُوتُوا۟ ٱلْكِتَٰبَ مِن قَبْلِكُمْ وَمِنَ ٱلَّذِينَ أَشْرَكُوٓا۟ أَذًۭى كَثِيرًۭا ۚ وَإِن تَصْبِرُوا۟ وَتَتَّقُوا۟ فَإِنَّ ذَٰلِكَ مِنْ عَزْمِ ٱلْأُمُورِ ﴿١٨٦﴾
You will surely be tested in your possessions and in yourselves. And you will surely hear from those who were given the Scripture before you and from those who associate others with Allah much abuse. But if you are patient and fear Allah - indeed, that is of the matters [worthy] of determination.
Believers are told to answer abuse with patience and taqwa.
Sahih Muslim 1869
Sahih... لا تسافروا بالقرآن فإني لا آمن أن يناله العدو ...
"Do not travel with the Quran, for I do not feel safe that the enemy will get hold of it."
Muslims should protect the mushaf from places where it may be abused.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Jihad, chapter on the apostate (Bab al-Murtad). Lists throwing the mushaf in filth out of contempt among the acts that are disbelief.
- al-Fatawa al-Hindiyya, Kitab al-Siyar, chapter on the rulings of apostates. Treats deliberate insult of the Quran as disbelief.
- al-Itqan (al-Suyuti), Chapter on the collection of the Quran. Records the burning of extra copies under Uthman as an act of protection.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.