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