Sayyiduna Ibn Umar (may Allah be pleased with them both) relates: We used to pass a definitive judgement (of Hell) on the killer, until this verse was revealed: "Indeed, Allah will not forgive that partners be associated with Him, and He will forgive what is less than that for whomever He wills" [Surah al-Nisa: 48].
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13307