Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) said: "Whoever kills a protected non-Muslim (mu'ahad) without right will not even smell the fragrance of Paradise, though its fragrance can be found from a distance of forty years."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسامة / 16482