Musannaf Ibn Abi Shayba 18:1223

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, he said: Al-Hasan ibn 'Amr narrated to us, from Mujahid, from 'Abdullah ibn 'Amr, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever kills a person who is a *mu'ahid* (a non-Muslim under a covenant of protection) without right, he shall not smell the fragrance of Paradise, even though its fragrance is found from a distance of forty years' journey."