´Narrated Ad-Dhahak:` 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."
Hadith Referenceسنن ترمذي / كتاب الإيمان عن رسول الله صلى الله عليه وسلم / 2636
´Narrated Ibn 'Umar:` Ibn 'Umar narrated that the Prophet (ﷺ) said: "Whoever says to his brother 'disbeliever' then it will have settled upon one of them." Sahih
Hadith Referenceسنن ترمذي / كتاب الإيمان عن رسول الله صلى الله عليه وسلم / 2637