Ibn 'Umar narrated that the Prophet (ﷺ) said: "Whoever says to his brother 'disbeliever' then it will have settled upon one of them." Sahih
Jami` at-Tirmidhi Book 40, Hadith 32
← Previous Hadith 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."

Jami` at-Tirmidhi Book 40, Hadith 31 Next Hadith → from Ubadah bin As-Samit, he said: "I entered upon him while he was dying, so I cried, and he said: "There now, why are you crying? For by Allah, if I am a martyr , then I will intercede for you, and if I can I will benefit you,' then he said: 'By Allah! There is no Hadith which I heard from the Messenger of Allah (ﷺ) which is good for you but I narrate it to you today, while I am near death. I heard the Messenger of Allah (ﷺ) say: "Whoever testifies to La Ilaha Illallah and that Muhammad is the Messenger of Allah, then Allah has forbidden the fire for him."

Jami` at-Tirmidhi Book 40, Hadith 33