Sunan an-Nasa'i 3984
سنن النسائي#3984SahihIt 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiIsnaad Sahih
In-book reference: 37:19