The Prophet ﷺ said, "Whoever throws himself off a mountain and kills himself will be in the fire of Hell, throwing himself down within it, abiding there eternally and forever. Whoever swallows poison and kills himself will have his poison in his hand, sipping it in the fire of Hell, abiding there eternally and forever. Whoever kills himself with a piece of iron will have his iron in his hand, stabbing himself with it in his belly in the fire of Hell, abiding there eternally and forever."
عن النبي ﷺ قال: من تردى من جبل فقتل نفسه فهو في نار جهنم يتردى فيه خالدا مخلدا فيها أبدا، ومن تحسى سما فقتل نفسه فسمه في يده يتحساه في نار جهنم نار جهنم خالدا مخلدا فيها أبدا، ومن قتل نفسه بحديدة فحديدته في يده يجأ بها في بطنه في خالدا مخلدا فيها أبدا.
Bukhari narrated it in "Medicine" (5778) and Muslim in "Faith" (109), both through Khalid bin Harith, who said, Shuayb narrated to us, from Sulayman (that is, Amash), who said, I heard Zakwan narrating from Abu Hurairah, and he mentioned it. Tirmidhi narrated it (2042, 2043) through several chains from Amash with his isnad, similarly. He said, "Muhammad bin Ajlan narrated from Saeed Maqburi, from Abu Hurairah, from the Prophet ﷺ, who said, 'Whoever kills himself with poison will be tormented with it in the fire of Hell,' without mentioning 'abiding therein forever.' Abu Zinad likewise narrated it from Araj, from Abu Hurairah, from the Prophet ﷺ." He said, "This is more sahih, because the reports only indicate that the people of monotheism are punished in the Fire and then brought out of it. It is not mentioned that they abide in it forever." End of quote.I say, the hadith of Abu Zinad was recorded by Bukhari (1365) from Abu Yaman, who said, Shuayb informed us, from him, from Araj, from Abu Hurairah, who said, the Messenger of Allah said, "The one who strangles himself will be strangling himself in the Fire, and the one who stabs himself will be stabbing himself in the Fire." Muhammad bin Ajlan also narrated something similar from Abu Zinad, from Araj, from Abu Hurairah. Imam Ahmad (9618) and Ibn Hibban (5987) narrated it through his chain, and Tahawi narrated it in his Mushkil (195) through Malik bin Anas, from Abu Zinad with his isnad. They added in their version, "and the one who throws himself down will be throwing himself down in the Fire," meaning that he casts himself into deadly situations, such as falling from a mountain or doing something similar.As for the meaning of his saying, "he will be in the fire of Hell, abiding therein forever," Nawawi said in his commentary on Muslim that there are several views on this. The first is that it applies to someone who did that while considering it permissible, despite knowing it was forbidden. Such a person is a disbeliever, and this is his punishment. The second is that what is meant by "abiding forever" is a long duration and a prolonged stay, not literal eternity, just as one says, "May Allah perpetuate the sultan's rule." The third is that this is his deserved punishment, but Allah showed generosity and informed us that whoever dies a Muslim does not abide forever in the Fire. End of quote.The proof that one who kills himself does not become a disbeliever is the following hadith.