Musnad Ahmad 1018:118

Abu al-Nadr narrated to us, al-Ashja‘i narrated to us, from Sufyan, from al-A‘mash, from Hilal ibn Yasaf, from a man, from the Prophet, peace and blessings of Allah be upon him, that he said: "There will be a people who have a covenant. Whoever kills a man from among them will not smell the fragrance of Paradise, even though its fragrance can be found from a distance of seventy years."