Messenger of Allah ﷺ said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief).  

قَالَ رَسُولُ اللَّهِ ﷺ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ  

That he heard the Prophet ﷺ saying, "If somebody accuses another of Fusuq (by calling him 'Fasiq' i.e. a wicked person) or accuses him of Kufr, such an accusation will revert to him (i.e. the accuser) if his companion (the accused) is innocent."  

أَنَّهُ سَمِعَ النَّبِيَّ ﷺ يَقُولُ لاَ يَرْمِي رَجُلٌ رَجُلاً بِالْفُسُوقِ وَلاَ يَرْمِيهِ بِالْكُفْرِ إِلاَّ ارْتَدَّتْ عَلَيْهِ إِنْ لَمْ يَكُنْ صَاحِبُهُ كَذَلِكَ  

Messenger of Allah ﷺ was neither a Fahish (one who had a bad tongue) nor a Sabbaba (one who abuses others) and he used to say while admonishing somebody, "What is wrong with him? May dust be on his forehead!"  

لَمْ يَكُنْ رَسُولُ اللَّهِ ﷺ فَاحِشًا وَلاَ لَعَّانًا وَلاَ سَبَّابًا كَانَ يَقُولُ عِنْدَ الْمَعْتَبَةِ مَا لَهُ تَرِبَ جَبِينُهُ  

who was one of the companions of the tree (who gave the pledge of allegiance to the Prophet ﷺ underneath the tree at Ḥudaybiyyah) that the Messenger of Allah ﷺ said, "Whoever swears by a religion other than Islam then he is really so. A child of Adam is not bound to fulfill a vow to Allah about a thing which he does not possess."

"If someone commits suicide with anything in this world, he will be tortured with it on the Day of Resurrection. If somebody curses a believer then his sin will be as if he killed him. And whoever accuses a believer of kufr (disbelief) then it is as if he killed him."  

وَكَانَ مِنْ أَصْحَابِ الشَّجَرَةِ حَدَّثَهُ أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «مَنْ حَلَفَ عَلَى مِلَّةٍ غَيْرِ الإِسْلاَمِ فَهْوَ كَمَا قَالَ؛ وَلَيْسَ عَلَى ابْنِ آدَمَ نَذْرٌ فِيمَا لاَ يَمْلِكُ؛»

«وَمَنْ قَتَلَ نَفْسَهُ بِشَىْءٍ فِي الدُّنْيَا عُذِّبَ بِهِ يَوْمَ الْقِيَامَةِ؛ وَمَنْ لَعَنَ مُؤْمِنًا فَهْوَ كَقَتْلِهِ؛ وَمَنْ قَذَفَ مُؤْمِنًا بِكُفْرٍ فَهْوَ كَقَتْلِهِ۔»  

A man from the companions of the Prophet ﷺ said, "Two men abused each other in front of the Prophet ﷺ and one of them became angry and his anger became so intense that his face became swollen and changed. The Prophet ﷺ said, "I know a word the saying of which will cause him to relax if he does say it." Then a man went to him and informed him of the statement of the Prophet ﷺ and said, "Seek refuge with Allah from Satan." On that, angry man said, 'Do you find anything wrong with me? Am I insane? Go away!"  

اسْتَبَّ رَجُلاَنِ عِنْدَ النَّبِيِّ ﷺ فَغَضِبَ أَحَدُهُمَا فَاشْتَدَّ غَضَبُهُ حَتَّى انْتَفَخَ وَجْهُهُ وَتَغَيَّرَ فَقَالَ النَّبِيُّ ﷺ إِنِّي لأَعْلَمُ كَلِمَةً لَوْ قَالَهَا لَذَهَبَ عَنْهُ الَّذِي يَجِدُ فَانْطَلَقَ إِلَيْهِ الرَّجُلُ فَأَخْبَرَهُ بِقَوْلِ النَّبِيِّ ﷺ وَقَالَ تَعَوَّذْ بِاللَّهِ مِنَ الشَّيْطَانِ فَقَالَ أَتُرَى بِي بَأْسٌ أَمَجْنُونٌ أَنَا اذْهَبْ  

Messenger of Allah ﷺ went out to inform the people about the (date of the Night of decree (Al-Qadr). There happened a quarrel between two Muslim men. The Prophet ﷺ said, "I came out to inform you about the Night of Al-Qadr, but as so-and-so and so-and-so quarrelled, so the news about it had been taken away; and may be it was better for you. So look for it in the ninth, the seventh, or the fifth (of the last ten days of Ramadan).  

خَرَجَ رَسُولُ اللَّهِ ﷺ لِيُخْبِرَ النَّاسَ بِلَيْلَةِ الْقَدْرِ فَتَلاَحَى رَجُلاَنِ مِنَ الْمُسْلِمِينَ قَالَ النَّبِيُّ ﷺ خَرَجْتُ لأُخْبِرَكُمْ فَتَلاَحَى فُلاَنٌ وَفُلاَنٌ وَإِنَّهَا رُفِعَتْ وَعَسَى أَنْ يَكُونَ خَيْرًا لَكُمْ فَالْتَمِسُوهَا فِي التَّاسِعَةِ وَالسَّابِعَةِ وَالْخَامِسَةِ  

I saw Abu Dhar wearing a Burd (garment) and his slave too was wearing a Burd, so I said (to Abu Dhar), "If you take this (Burda of your slave) and wear it (along with yours), you will have a nice suit (costume) and you may give him another garment." Abu Dhar said, "There was a quarrel between me and another man whose mother was a non-Arab and I called her bad names. The man mentioned (complained about) me to the Prophet. The Prophet ﷺ said, "Did you abuse so-and-so?" I said, "Yes" He said, "Did you call his mother bad names?" I said, "Yes". He said, "You still have the traits of (the Pre-lslamic period of) ignorance." I said. "(Do I still have ignorance) even now in my old age?" He said, "Yes, they (slaves or servants) are your brothers, and Allah has put them under your command. So the one under whose hand Allah has put his brother, should feed him of what he eats, and give him dresses of what he wears, and should not ask him to do a thing beyond his capacity. And if at all he asks him to do a hard task, he should help him therein."  

رَأَيْتُ عَلَيْهِ بُرْدًا وَعَلَى غُلاَمِهِ بُرْدًا فَقُلْتُ لَوْ أَخَذْتَ هَذَا فَلَبِسْتَهُ كَانَتْ حُلَّةً وَأَعْطَيْتَهُ ثَوْبًا آخَرَ فَقَالَ كَانَ بَيْنِي وَبَيْنَ رَجُلٍ كَلاَمٌ وَكَانَتْ أُمُّهُ أَعْجَمِيَّةً فَنِلْتُ مِنْهَا فَذَكَرَنِي إِلَى النَّبِيِّ ﷺ فَقَالَ لِي أَسَابَبْتَ فُلاَنًا قُلْتُ نَعَمْ قَالَ أَفَنِلْتَ مِنْ أُمِّهِ قُلْتُ نَعَمْ قَالَ إِنَّكَ امْرُؤٌ فِيكَ جَاهِلِيَّةٌ قُلْتُ عَلَى حِينِ سَاعَتِي هَذِهِ مِنْ كِبَرِ السِّنِّ قَالَ نَعَمْ هُمْ إِخْوَانُكُمْ جَعَلَهُمُ اللَّهُ تَحْتَ أَيْدِيكُمْ فَمَنْ جَعَلَ اللَّهُ أَخَاهُ تَحْتَ يَدِهِ فَلْيُطْعِمْهُ مِمَّا يَأْكُلُ وَلْيُلْبِسْهُ مِمَّا يَلْبَسُ وَلاَ يُكَلِّفُهُ مِنَ الْعَمَلِ مَا يَغْلِبُهُ فَإِنْ كَلَّفَهُ مَا يَغْلِبُهُ فَلْيُعِنْهُ عَلَيْهِ