It was narrated from Abu Hurairah that the Prophet (ﷺ) said:
"Whoever kills a Mu'ahid who has the protection of Allah and the protection of his Messenger, will not smell the fragrance of Paradise, even though its fragrance may be detected from a distance of Seventy years." (ID)
English reference (ID): Vol. 3, Book 21, Hadith 2687 (ID)
Arabic reference (ID): Book 21, Hadith 2789 (ID)