Sunan an-Nasa'i, Hadith 3984 ⓘ
It was narrated that Abu Idris said: "I heard Mu'awiyah delivering the Khutbah, and he narrated a few Hadiths from the Messenger of Allah [SAW]." He said: "I heard him delivering a Khutbah and he said: 'I heard the Messenger of Allah [SAW] say: Every sin may be forgiven by Allah except a man who kills a believer deliberately, or a man who dies as a disbeliever.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا صَفْوَانُ بْنُ عِيسَى، عَنْ ثَوْرٍ، عَنْ أَبِي عَوْنٍ، عَنْ أَبِي إِدْرِيسَ، قَالَ سَمِعْتُ مُعَاوِيَةَ، يَخْطُبُ - وَكَانَ قَلِيلَ الْحَدِيثِ - عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ سَمِعْتُهُ يَخْطُبُ يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " كُلُّ ذَنْبٍ عَسَى اللَّهُ أَنْ يَغْفِرَهُ إِلاَّ الرَّجُلُ يَقْتُلُ الْمُؤْمِنَ مُتَعَمِّدًا أَوِ الرَّجُلُ يَمُوتُ كَافِرًا " .
also referenced under: Book 37, Hadith 19 (In-Book) & Volume 5, Book 37, Hadith 3989 (DarusSalam)