It was narrated from 'Abdullah bin 'Amr that the Messenger of Allah (ﷺ) said:
"Whoever kills a Mu'ahid, will not smell the fragrance of Paradise, even though its fragrance may be detected from a distance of forty years." (IT)
English reference (IT): Vol. 3, Book 21, Hadith 2686 (IT)
Arabic reference (IT): Book 21, Hadith 2788 (IT)