that the Prophet ﷺ said: "It is not for a slave (of Allah) to vow about something he does not possess, and cursing a believer is like killing him, and whoever accuses a believer of disbelief, then it is like he has killed him, and whoever kills himself with something, then Allah will punish him with whatever he killed himself with on the Day of Judgement."  

عَنِ النَّبِيِّ ﷺ قَالَ لَيْسَ عَلَى الْعَبْدِ نَذْرٌ فِيمَا لاَ يَمْلِكُ وَلاَعِنُ الْمُؤْمِنِ كَقَاتِلِهِ وَمَنْ قَذَفَ مُؤْمِنًا بِكُفْرٍ فَهُوَ كَقَاتِلِهِ وَمَنْ قَتَلَ نَفْسَهُ بِشَيْءٍ عَذَّبَهُ اللَّهُ بِمَا قَتَلَ بِهِ نَفْسَهُ يَوْمَ الْقِيَامَةِ وَفِي الْبَابِ عَنْ أَبِي ذَرٍّ وَابْنِ عُمَرَ  

Ibn 'Umar narrated that the Prophet ﷺ said: "Whoever says to his brother 'disbeliever' then it will have settled upon one of them." Sahih  

عَنِ النَّبِيِّ ﷺ قَالَ أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ كَافِرٌ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا هَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ وَمَعْنَى قَوْلِهِ بَاءَ يَعْنِي أَقَرَّ