Musnad Ahmad 616:1

Waki' narrated to us, my father narrated to us, from Mansur, from Hilal ibn Yasaf, from al-Qasim ibn Mukhaymirah, from a man among the companions of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Whoever kills a man from the people of the covenant (ahl al-dhimmah) will not find the scent of Paradise, and indeed its scent is found from a distance of seventy years."