Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever unjustly kills a person under covenant (mu'ahad) will not attain the fragrance of Paradise, although its fragrance can be perceived from a distance of forty years' travel."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18732