Short answer
Major sins (kaba'ir) are sins the Quran or Sunnah threatens with Hell, a curse, Allah's anger or a fixed punishment. The Prophet ﷺ named seven destroyers: shirk, magic, unjust killing, consuming riba, consuming an orphan's wealth, fleeing the battlefield and slandering chaste women. Others include disobeying parents, false testimony, zina, drinking alcohol and abandoning prayer.
Hanafi scholars, like the others, divide sins into major (kaba'ir) and minor (sagha'ir). Major sins need sincere repentance, and they do not take a believer out of Islam unless he holds them lawful. There is no fixed number: al-Dhahabi's well-known book lists seventy.
The four schools agree on this.
Explanation
Allah says: "If you avoid the major sins which you are forbidden, We will remove from you your lesser sins and admit you to a noble entrance" (4:31). So the Quran itself makes the division, and promises that avoiding the big sins brings forgiveness of the small.
The Prophet ﷺ said: "Avoid the seven destroyers." When asked what they are, he named associating partners with Allah, magic, killing a soul Allah has made sacred except by right, consuming riba, consuming an orphan's wealth, fleeing on the day of battle, and slandering chaste, unaware believing women. In another hadith he named the greatest of the major sins as shirk, disobedience to parents and false speech or testimony, repeating the last one so often that the Companions wished he would stop.
Other sins the scholars count as major include zina, drinking alcohol, theft, abandoning the obligatory prayer, refusing zakat, breaking the Ramadan fast without excuse, cutting ties of kinship, lying about the Prophet ﷺ, backbiting and arrogance. Is there an official order? Only the hadith that name the greatest; beyond that the scholars ranked them by the severity of the warning. A minor sin also becomes major when a person persists in it or treats it lightly.
No sin is too big for repentance. See the biggest sin in Islam, is a Muslim who commits a major sin still a Muslim?, minor sins and how to repent.
The evidence
Quran, Surah An-Nisaa 4:31
إِن تَجْتَنِبُوا۟ كَبَآئِرَ مَا تُنْهَوْنَ عَنْهُ نُكَفِّرْ عَنكُمْ سَيِّـَٔاتِكُمْ وَنُدْخِلْكُم مُّدْخَلًۭا كَرِيمًۭا ﴿٣١﴾
If you avoid the major sins which you are forbidden, We will remove from you your lesser sins and admit you to a noble entrance [into Paradise].
Divides sins into major and minor.
Quran, Surah An-Najm 53:32
ٱلَّذِينَ يَجْتَنِبُونَ كَبَٰٓئِرَ ٱلْإِثْمِ وَٱلْفَوَٰحِشَ إِلَّا ٱللَّمَمَ ۚ إِنَّ رَبَّكَ وَٰسِعُ ٱلْمَغْفِرَةِ ۚ هُوَ أَعْلَمُ بِكُمْ إِذْ أَنشَأَكُم مِّنَ ٱلْأَرْضِ وَإِذْ أَنتُمْ أَجِنَّةٌۭ فِى بُطُونِ أُمَّهَٰتِكُمْ ۖ فَلَا تُزَكُّوٓا۟ أَنفُسَكُمْ ۖ هُوَ أَعْلَمُ بِمَنِ ٱتَّقَىٰٓ ﴿٣٢﴾
Those who avoid the major sins and immoralities, only [committing] slight ones. Indeed, your Lord is vast in forgiveness. He was most knowing of you when He produced you from the earth and when you were fetuses in the wombs of your mothers. So do not claim yourselves to be pure; He is most knowing of who fears Him.
Praises those who avoid the major sins.
Sahih al-Bukhari 2766
Sahih... الشِّرْكُ بِاللَّهِ، وَالسِّحْرُ، وَقَتْلُ النَّفْسِ الَّتِي حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ ...
"Associating partners with Allah, magic, and killing a soul Allah has made sacred, except by right."
The start of the list of the seven destroying sins.
Where the ruling is recorded
- al-Kaba'ir (al-Dhahabi), Introduction and the list of seventy major sins. Defines a major sin by the threat attached to it and lists seventy.
- Radd al-Muhtar (Ibn Abidin), Book of testimony, on what makes a witness unreliable. Discusses the definition of a major sin in the Hanafi school.
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.