Verily the Messenger of Allah (ﷺ) said: He who took an oath on a religion other than Islam as a liar would become so as he said, and he who slaughtered himself with a thing would be slaughtered with that on the Day of Resurrection.
Sahih Muslim Book 1, Hadith 211
← Previous Hadith He who took deliberately a false oath on a religion other than Islam would become that which he had professed. And he who killed himself with anything Allah would torment him with that in the Fire of Hell.

Sahih Muslim Book 1, Hadith 210 Next Hadith → We participated in the Battle of Hunain along with the Messenger of Allah (ﷺ). He (the Holy Prophet) said about a man who claimed to be a Muslim that he was one of the denizens of the Fire (of Hell). When we were in the thick of the battle that man fought desperately and was wounded. It was said: Messenger of Allah, the person whom you at first called as the denizen of Fire fought desperately and died. Upon this the Messenger of Allah (ﷺ) remarked: He was doomed to the Fire (of Hell). Some men were on the verge of doubt (about his fate) when it was said that he was not dead but fatally wounded. When it was night he could not stand the (pain of his) wound and killed himself. The Apostle (ﷺ) was informed of that. He (the Holy Prophet) observed: Allah is Great, I bear testimony to the fact that I am the servant of Allah and His messenger. He then commanded Bilal to announce to the people that none but a Muslim would enter Paradise. Verily Allah helps this faith even by a sinful person.

Sahih Muslim Book 1, Hadith 212