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." (PT)
English reference (PT): Vol. 3, Book 21, Hadith 2686 (PT)
Arabic reference (PT): Book 21, Hadith 2788 (PT)