✅Please pay attention to the following narration:

عَبَّادِ بْنِ کَثِیرٍ قَالَ: سَأَلْتُ أَبَا جَعْفَرٍ عَنِ الْکَبَائِرِ فَقَالَ کُلُّ شَیْ ءٍ أَوْعَدَ اللَّهُ عَلَیْهِ النَّارَ

📚Thawab al-A'mal, page 209

Translation: 'Abbad said: I asked Imam Baqir (peace be upon him) about the major sins; the Imam said: Anything for which Allah has promised a threat of fire (is a major sin).

✅This hadith provides a comprehensive definition of a major sin, although some are greater than others. Among them, seven are named as the greatest of the major sins, which are mentioned in the following narration:

أَبِی عَبْدِ اللَّهِ قَالَ أَکْبَرُ الْکَبَائِرِ سَبْعٌ الشِّرْکُ بِاللَّهِ الْعَظِیمِ وَ قَتْلُ النَّفْسِ الَّتِی حَرَّمَ اللَّهُ عَزَّ وَ جَلَّ وَ أَکْلُ أَمْوَالِ الْیَتَامَی وَ عُقُوقُ الْوَالِدَیْنِ وَ قَذْفُ الْمُحْصَنَاتِ وَ الْفِرَارُ مِنَ الزَّحْفِ وَ إِنْکَارُ مَا أَنْزَلَ اللَّهُ عَزَّ وَ جَلَّ

📚Tafsir al-Ayyashi, vol. 1, page 237

Translation: Imam Sadiq (peace be upon him) said: The greatest of the major sins are seven: Associating partners with the Great Allah, killing a soul that Allah, the Exalted, has forbidden, consuming the wealth of orphans, harming and disobeying parents, slandering chaste women, fleeing from battle, and denying what Allah, the Exalted, has revealed.

✅Also, pay attention to the following narration:

فِیمَا کَتَبَ الرِّضَا لِلْمَأْمُونِ مِنْ شَرَائِعِ الدِّینِ وَ اجْتِنَابُ الْکَبَائِرِ وَ هِیَ قَتْلُ النَّفْسِ الَّتِی حَرَّمَ اللَّهُ عَزَّ وَ جَلَّ وَ الزِّنَا وَ السَّرِقَةُ وَ شُرْبُ الْخَمْرِ وَ عُقُوقُ الْوَالِدَیْنِ وَ الْفِرَارُ مِنَ الزَّحْفِ وَ أَکْلُ مَالِ الْیَتِیمِ ظُلْماً وَ أَکْلُ الْمَیْتَةِ وَ الدَّمِ وَ لَحْمِ الْخِنْزِیرِ وَ ما أُهِلَّ لِغَیْرِ اللَّهِ بِهِ مِنْ غَیْرِ ضَرُورَةٍ وَ أَکْلُ الرِّبَا بَعْدَ الْبَیِّنَةِ وَ السُّحْتِ وَ الْمَیْسِرُ وَ هُوَ الْقِمَارُ وَ الْبَخْسُ فِی الْمِکْیَالِ وَ الْمِیزَانِ وَ قَذْفُ الْمُحْصَنَاتِ وَ اللِّوَاطُ وَ شَهَادَةُ الزُّورِ وَ الْیَأْسُ مِنْ رَوْحِ اللَّهِ وَ الْأَمْنُ مِنْ مَکْرِ اللَّهِ وَ الْقُنُوطُ مِنْ رَحْمَةِ اللَّهِ وَ مَعُونَةُ الظَّالِمِینَ وَ الرُّکُونُ إِلَیْهِمْ وَ الْیَمِینُ الْغَمُوسُ وَ حَبْسُ الْحُقُوقِ مِنْ غَیْرِ عُسْرٍ وَ الْکَذِبُ وَ الْکِبْرُ وَ الْإِسْرَافُ وَ التَّبْذِیرُ وَ الْخِیَانَةُ وَ الِاسْتِخْفَافُ بِالْحَجِّ وَ الْمُحَارَبَةُ لِأَوْلِیَاءِ اللَّهِ تَعَالَی وَ الِاشْتِغَالُ بِالْمَلَاهِی وَ الْإِصْرَارُ عَلَی الذُّنُوبِ

📚Uyun al-Akhbar, vol. 2, page 127

Translation: Among what Imam Reza (peace be upon him) wrote to Ma'mun (may he be cursed) concerning the laws of religion, it is stated: And abstaining from the major sins, which are: killing someone whom Allah has forbidden, fornication, theft, drinking wine, harming and disobeying parents, fleeing from jihad, consuming the wealth of an orphan unjustly, consuming dead meat, blood, pork, and what is slaughtered for idols without necessity, consuming usury after a clear prohibition, and (consuming) illicit and forbidden gains, and gambling, and giving short measure and weight, and slandering chaste women, and sodomy, and bearing false witness, and despairing of divine mercy, and assisting and aiding the oppressors, and inclining towards them, and taking a false oath, and withholding the rights of others without being in hardship, and lying, and arrogance, and extravagance and wasteful spending, and treachery, and belittling the Hajj, and fighting with the friends of Allah, the Most High, and being preoccupied with pastimes that distract from the remembrance of Allah, and persisting in sins.

الصَّادِقِ عَنْ أَبِیهِ قَالَ: الْحَیْفُ فِی الْوَصِیَّةِ مِنَ الْکَبَائِرِ یَعْنِی الظُّلْمَ فِیهَا

📚Qurb al-Isnad, page 34

Translation: Imam Baqir (peace be upon him) said: Oppression in a will, meaning injustice in it, is one of the major sins.

أَبِی عَبْدِ اللَّهِ قَالَ: الْکَذِبُ عَلَی اللَّهِ عَزَّ وَ جَلَّ وَ عَلَی رَسُولِهِ وَ عَلَی الْأَوْصِیَاءِ مِنَ الْکَبَائِرِ

📚Thawab al-A'mal, page 239

Translation: Imam Sadiq (peace be upon him) said: Lying about Allah, the Exalted, and His Messenger, and His successors, is one of the major sins.

عَنِ الرِّضَا فِی قَوْلِ اللَّهِ تَبَارَکَ وَ تَعَالَی إِنْ تَجْتَنِبُوا کَبائِرَ ما تُنْهَوْنَ عَنْهُ نُکَفِّرْ عَنْکُمْ سَیِّئاتِکُمْ قَالَ مَنِ اجْتَنَبَ مَا أَوْعَدَ اللَّهُ عَلَیْهِ النَّارَ إِذَا کَانَ مُؤْمِناً کَفَّرَ عَنْهُ سَیِّئَاتِهِ

📚Thawab al-A'mal, page 117

Translation: Imam Reza (peace be upon him) said regarding the verse 'If you avoid the major sins which you are forbidden, We will remove your evil deeds from you': He who avoids what Allah, the Blessed and Exalted, has promised the threat of fire for, if he is a believer, He will cover his evil deeds.

✅Based on the first hadith, any sin for which a threat of fire has been promised in the Quran and narrations is a major sin.