On what did the scholars of Ahlus-Sunnah base their statement that the eternity of a suicide in Hell is not like the eternal damnation of disbelievers, even though there are prophetic hadiths mentioning their eternal stay in Hell, such as the hadith of Abu Hurairah and the hadith of Jundub bin Abdullah?

The methodology of Ahl al-Sunnah (definition) wa al-Jama'ah involves understanding sacred texts by integrating them, giving precedence to the restricted over the unrestricted, the specific over the general, the abrogating over the abrogated, and the authentic over the weak, and referring the ambiguous to the clear. According to Ahl al-Sunnah, no one is considered to have committed an act of disbelief that expels them from Islam except for those who come with something that nullifies their Islam, whether in belief, word, or deed. Committing sins and transgressions does not fall under this, unless one considers them permissible.

A Muslim killing himself does not expel him from Islam nor does it necessitate eternal damnation in Hellfire. This is supported by the following evidence:

1. Clear texts stating that if a Muslim meets Allah with every sin except major shirk (definition), their sins are forgivable. Allah Almighty says: (Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills) (An-Nisa: 48). Killing oneself is not shirk. 2. The practical reality concerning one who kills himself in the purified Sharia (definition): He is washed, prayed over, buried in Muslim cemeteries, and inherits. If he were a disbeliever, these rulings would not apply to him. The Prophet's ﷺ abstention from praying over him was a form of deterrence and severity, not because he was a disbeliever, and the Companions did pray over him.

The eternity mentioned in the hadiths for one who kills himself is a qualified eternity, meaning a lengthy stay in Hellfire that eventually ends, not an eternal stay like that of disbelievers and polytheists. The prohibition of Paradise for him is a temporary prohibition (he will not enter it with the first entrants), or it refers to a higher level of Paradise, or if he considered the act permissible.