Musnad Ahmad 435:80

Safwan ibn Isa narrated to us, he said: Thawr ibn Yazid informed us, from Abu Awn, from Abu Idris, he said: I heard Muawiyah—and he was one who spoke little about the Messenger of Allah, peace and blessings of Allah be upon him—he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, while he was saying: "Every sin, it is possible that Allah will forgive it, except for a man who dies as a disbeliever, or a man who kills a believer intentionally."