Musnad Ahmad 830:31

Yahya narrated to us, from 'Uyaynah, my father narrated to me, from Abu Bakrah, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever kills a *mu'ahid* (a non-Muslim under a covenant of protection) without his right (i.e., without a legal justification), Allah has forbidden Paradise to him, that he should smell its fragrance."