Musnad Ahmad 830:142

'Affan narrated to us, Hammad bin Salamah narrated to us, 'Ali bin Zayd informed us, from 'Abd al-Rahman bin Abi Bakrah, from his father, that the Messenger of Allah (ﷺ) said: "Whoever kills a person under a covenant (mu'ahadah) without a rightful cause, he shall not find the fragrance of Paradise, even though its fragrance can be found from a distance of a hundred years' journey."