I heard the Messenger of Allah ﷺ say, "Every sin may be forgiven by Allah, except for a man who dies as a disbeliever, or a man who kills a believer intentionally."
سمعت رسول الله ﷺ يقول: كل ذنب عسى الله أن يغفره إلا الرجل يموت كافرا، أو الرجل يقتل مؤمنا متعمدا.
Nasai (7/81), Ahmad (16907), and Hakim (4/351), who graded it sahih, all narrated it through the route of Safwan bin Isa, who said, Thawr bin Yazid informed us, from Abu Awn, from Abu Idris, who said, I heard Muawiya, who narrated little from the Messenger of Allah ﷺ, say... and he mentioned it, with the wording belonging to Ahmad.Its chain of transmission is hasan because of Abu Awn Ansari, for his hadith is hasan. For more, see the book on legal punishments.