Answer
✅By paying careful attention to the following hadiths, we will understand the gravity of the sin of backbiting and some of its limits:
نَوْفٍ الْبِکَالِیِّ قَالَ فَقُلْتُ لَهُ یَا أَمِیرَ الْمُؤْمِنِینَ عِظْنِی فَقَالَ.. إِلَی أَنْ قَالَ قُلْتُ زِدْنِی قَالَ اجْتَنِبِ الْغِیبَةَ فَإِنَّهَا إِدَامُ کِلَابِ النَّارِ ثُمَّ قَالَ یَا نَوْفُ کَذَبَ مَنْ زَعَمَ أَنَّهُ وُلِدَ مِنْ حَلَالٍ وَ هُوَ یَأْکُلُ لُحُومَ النَّاسِ بِالْغِیبَةِ
📚 Amali Saduq, p. 209
Translation: Nawf al-Bakali said: Amir al-Mu'minin, peace be upon him, said: Stay away from backbiting, for it is the stew of the dogs of Hell. Then he said: O Nawf, he who thinks that he was born from a lawful union lies while he (constantly) eats the flesh of people through backbiting.
عَنِ الصَّادِقِ فِی حَدِیثٍ وَ لَقَدْ حَدَّثَنِی أَبِی عَنْ أَبِیهِ عَنْ آبَائِهِ عَنْ رَسُولِ اللَّهِ قَالَ مَنِ اغْتَابَ مُؤْمِناً بِمَا فِیهِ لَمْ یَجْمَعِ اللَّهُ بَیْنَهُمَا فِی الْجَنَّةِ أَبَداً وَ مَنِ اغْتَابَ مُؤْمِناً بِمَا لَیْسَ فِیهِ فَقَدِ انْقَطَعَتِ الْعِصْمَةُ بَیْنَهُمَا وَ کَانَ الْمُغْتَابُ فِی النَّارِ خَالِداً فِیهَا وَ بِئْسَ الْمَصِیر
📚 Amali Saduq, p. 103
Translation: Imam Sadiq, peace be upon him, said in a hadith: Whoever backbites a believer about something that is in him, Allah will never gather them together in Paradise, and whoever backbites him about something that is not in him, the protection between the two has been cut off and the backbiter is in the Fire forever, which is a bad place to return to.
عَنِ الصَّادِقِ جَعْفَرِ بْنِ مُحَمَّدٍ قَالَ إِنَّ مِنَ الْغِیبَةِ أَنْ تَقُولَ فِی أَخِیکَ مَا سَتَرَهُ اللَّهُ عَلَیْهِ وَ إِنَّ مِنَ الْبُهْتَانِ أَنْ تَقُولَ فِی أَخِیکَ مَا لَیْسَ فِیهِ
📚 Amali Saduq, p. 337
Translation: Imam Sadiq, peace be upon him, said: Indeed, one of the types of backbiting is that you mention something about your brother that Allah had concealed for him, and indeed, one of the types of slander is that you say something about your brother that is not in him.
عَبْدِ الرَّحْمَنِ بْنِ سَیَابَةَ قَالَ سَمِعْتُ أَبَا عَبْدِ اللَّهِ یَقُولُ الْغِیبَةُ أَنْ تَقُولَ فِی أَخِیکَ مَا سَتَرَهُ اللَّهُ عَلَیْهِ وَ أَمَّا الْأَمْرُ الظَّاهِرُ مِثْلَ الْحِدَّةِ وَ الْعَجَلَةِ فَلَا وَ الْبُهْتَانُ أَنْ تَقُولَ فِیهِ مَا لَیْسَ فِیهِ
📚 Al-Kafi, vol. 2, p. 358
Translation: Imam Sadiq, peace be upon him, said: Backbiting is that you mention about your brother that which Allah has concealed for him, but manifest matters like being bad-tempered and hasty are not backbiting, and slander is that you say something that is not in him.
یَحْیَی الْأَزْرَقَ قَالَ قَالَ لِی أَبُو الْحَسَنِ مَنْ ذَکَرَ رَجُلًا مِنْ خَلْفِهِ بِمَا هُوَ فِیهِ مِمَّا عَرَفَهُ النَّاسُ لَمْ یَغْتَبْهُ وَ مَنْ ذَکَرَهُ مِنْ خَلْفِهِ بِمَا هُوَ فِیهِ مِمَّا لَا یَعْرِفُهُ النَّاسُ اغْتَابَهُ وَ مَنْ ذَکَرَهُ بِمَا لَیْسَ فِیهِ فَقَدْ بَهَتَهُ
📚 Al-Kafi, vol. 2, p. 358
Translation: Yahya said: Imam Kazim, peace be upon him, said to me: Whoever mentions a person behind his back with something that is in him and people know that matter, he has not backbitten, and whoever mentions him with something that is in him but people do not know it, he has backbitten, and whoever mentions him with something that is not in him, he has slandered him.
عَنْ أَبِی ذَرٍّ عَنِ النَّبِیِّ فِی وَصِیَّةٍ لَهُ قَالَ یَا أَبَا ذَرٍّ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَ قِتَالُهُ کُفْرٌ وَ أَکْلُ لَحْمِهِ مِنْ مَعَاصِی اللَّهِ وَ حُرْمَةُ مَالِهِ کَحُرْمَةِ دَمِهِ قُلْتُ یَا رَسُولَ اللَّهِ وَ مَا الْغِیبَةُ قَالَ ذِکْرُکَ أَخَاکَ بِمَا یَکْرَهُ
📚 Amali Sheikh Tusi, p. 537
Translation: The Messenger of Allah, peace be upon him and his family, said in his will to Abu Dharr: O Abu Dharr, insulting a Muslim is a transgression, and fighting him is disbelief, and eating his flesh is from the sins against Allah, and the sanctity of his property is like the sanctity of his blood; I said: O Messenger of Allah, what is backbiting? He said: It is that you mention your brother with something that he dislikes.
✅According to the above narrations, backbiting a believing or Muslim brother about matters that Allah has concealed for him and mentioning matters that he dislikes to be mentioned is backbiting and has great condemnation and punishment. It is necessary for us to repent for the past instances and never again in the future become stained with this sin.
To know about those for whom backbiting is permissible, as well as the way to repent from backbiting and to see other condemnations and punishments for backbiting and the necessity of rejecting backbiting and... refer to 📚 Jami' Ahadith al-Shi'ah, vol. 20, pp. 518-544.