Questioner: His saying عليه الصلاة والسلام as occurs in Sahih Muslim, “Whoever kills himself with a piece of iron will have that iron in his hand, thrusting it into his belly in the Fire of Hell forever and ever,” the hadith. What kind of eternization is it? And does it necessitate disbelief?
Al-Albaani: Yes, what is apparent from the hadith is that it is talking about someone who holds suicide to be permissible, such a person will be in it forever and ever as is described in the hadith.
And we hold disbelief to be of two types as the people of knowledge and verification say: kufr in belief and kufr in actions. So whoever performs an act of the unbelievers and acknowledges the mistake of that action, he believed that he made a mistake in following the Legislation but his desires overcame him and the soul which is a persistent enjoiner of evil overcame him–then his disbelief is in action. As for if he coupled that with holding it to be permissible in his heart then that is kufr in belief and with that a Muslim leaves the religion.
So [a hadith] like this is taken to refer to the one whose kufr was in belief, because no one except an unbelieving polytheist who associates partners with Allaah تبارك وتعالى will abide in the Fire forever.
Questioner: Okay, O Shaikh, where do we take the fact about holding it to be permissible, that when he holds it to be permissible … from the apparent meaning of the hadith?
Al-Albaani: From the noble verse, “Indeed, Allaah does not forgive association with Him, but He forgives what is less than that for whom He wills,” [Nisaa 4:48] and from the description of this punishment, because no-one who has a speck of faith will abide in the Fire forever.
Al-Hudaa wan-Noor, 300.