It is narrated from a Companion (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: Whoever kills a dhimmi (a non-Muslim under Muslim protection) will not even smell the fragrance of Paradise, though its fragrance can be perceived from a distance of seventy years.
Hadith Referenceمسند احمد / مسند الشاميين / 18072
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد حسن