Sunan An-Nasa'i

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