Chapter 37: The Book of Fighting [The Prohibition of Bloodshed]
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.'"
Sunan an-Nasa'i 3984