Volume 4 · Book 53 · Hadith 391
Narrated 'Abdullah bin 'Amr: The Prophet said, "Whoever killed a person having a treaty with the Muslims, shall not smell the smell of Paradise though its smell is perceived from a distance of forty years."
— Sahih Bukhari, Volume 4, Book 53: One-fifth of Booty to the Cause of Allah (Khumus), Hadith #391