'Abdullah Ibn 'Umar (RAA) narrated that The Messenger of Allah (ﷺ) said:
"Whoever kills a Mu'ahid will not smell the fragrance of Paradise, even though its fragrance could be smelt at a distance of forty years." Related by Al-Bukhari.
English reference: Book 11, Hadith 1353
Arabic reference: Book 11, Hadith 1327