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 kills a protected non-Muslim (dhimmi) will not find the fragrance of Paradise, although its fragrance reaches from such-and-such a distance."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجزية / 18733