Sa'id ibn Malik reported that the Prophet, may Allah bless him and grant him peace, said, "Reviling a Muslim is deviant behaviour."  

عَنِ النَّبِيِّ ﷺ قَالَ سِبَابُ الْمُسْلِمِ فُسُوقٌ  

Anas said, "The Messenger of Allah, may Allah bless him and grant him peace, was neither coarse nor a curser nor a reviler, He used to say when he wanted to censure someone, 'What is wrong with him? May his brow be dusty!'"  

عَنْ أَنَسٍ قَالَ لَمْ يَكُنْ رَسُولُ اللهِ ﷺ فَاحِشًا وَلاَ لَعَّانًا وَلاَ سَبَّابًا كَانَ يَقُولُ عِنْدَ الْمَعْتَبَةِ مَا لَهُ تَرِبَ جَبِينُهُ  

'Abdullah reported that the Prophet, may Allah bless him and grant him peace, said, "Reviling a Muslim is deviant behaviour and killing him is disbelief."  

عَنِ النَّبِيِّ ﷺ سِبَابُ الْمُسْلِمِ فُسُوقٌ وَقِتَالُهُ كُفْرٌ  

Abu Dharr is reported as saying that he heard the Prophet, may Allah bless him and grant him peace, say, "If a man accuses another man of deviance or accuses him of disbelief, that accusation will come back on him if his companion is not as he said."  

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

Abu Dharr states that he heard the Prophet, may Allah bless him and grant him peace, say, "A person who knowingly claims a father other than his own has disbelieved. A person who claims to be from a people when he is not one of them will take his place in the Fire. A person who calls a man an unbeliever or says, 'Enemy of Allah,' when that is not the case will have that come back on him."  

سَمِعَ النَّبِيَّ ﷺ يَقُولُ مَنِ ادَّعَى لِغَيْرِ أَبِيهِ وَهُوَ يَعْلَمُ فَقَدْ كَفَرَ وَمَنِ ادَّعَى قَوْمًا لَيْسَ هُوَ مِنْهُمْ فَلْيَتَبَوَّأْ مَقْعَدَهُ مِنَ النَّارِ وَمَنْ دَعَا رَجُلاً بِالْكُفْرِ أَوْ قَالَ عَدُوُّ اللهِ وَلَيْسَ كَذَلِكَ إِلا حَارَتْ عَلَيْهِ  

Sulayman ibn Surad, one of the Companions of the Prophet, said, "Two men reviled one another in the presence of the Prophet, may Allah bless him and grant him peace, and one of them became angry. He became so angry that his face puffed out and changed colour. The Prophet, may Allah bless him and grant him peace, said, 'I know some words which will make what he feels depart if he says them.' The man came to him and told him what the Prophet, may Allah bless him and grant him peace, had said, He said, 'Seek refuge with Allah from the Accursed Shaytan.' He said, 'Do you think that there is something wrong with me? Am I mad? Leave!'"  

سَمِعْتُ سُلَيْمَانَ بْنَ صُرَدٍ رَجُلاً مِنْ أَصْحَابِ النَّبِيِّ ﷺ قَالَ اسْتَبَّ رَجُلاَنِ عِنْدَ النَّبِيِّ ﷺ فَغَضِبَ أَحَدُهُمَا فَاشْتَدَّ غَضَبُهُ حَتَّى انْتَفَخَ وَجْهُهُ وَتَغَيَّرَ فَقَالَ النَّبِيُّ ﷺ إِنِّي لَأَعْلَمُ كَلِمَةً لَوْ قَالَهَا لَذَهَبَ عَنْهُ الَّذِي يَجِدُ فَانْطَلَقَ إِلَيْهِ الرَّجُلُ فَأَخْبَرَهُ بِقَوْلِ النَّبِيِّ ﷺ وَقَالَ تَعَوَّذْ بِاللَّهِ مِنَ الشَّيْطَانِ الرَّجِيمِ وَقَالَ أَتَرَى بِي بَأْسًا أَمَجْنُونٌ أَنَا؟ اذْهَبْ  

'Abdullah said, "There is a veil from Allah Almighty between every two Muslims. When one of them says ugly words to his companion, he has rent the veil of Allah. When one of them tells the other, 'You are an unbeliever,' then one of them has disbelieved."  

عَنْ عَبْدِ اللهِ قَالَ مَا مِنْ مُسْلِمَيْنِ إِلاَّ بَيْنَهُمَا مِنَ اللهِ عَزَّ وَجَلَّ سِتْرٌ فَإِذَا قَالَ أَحَدُهُمَا لِصَاحِبِهِ كَلِمَةَ هَجْرٍ فَقَدْ خَرَقَ سِتْرَ اللهِ وَإِذَا قَالَ أَحَدُهُمَا لِلْآخَرِ أَنْتَ كَافِرٌ فَقَدْ كَفَرَ أَحَدُهُمَا