Musnad Ahmad 830:5

Waki’ and Abu ‘Abd al-Rahman narrated to us, they said: ‘Uyaynah narrated to us, from his father, from Abu Bakrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever kills a *mu’ahid* (a person under a covenant of protection) without his right (i.e., without a just cause), Allah will forbid Paradise to him." Abu ‘Abd al-Rahman said: "His right (*kunhuhu*): his due (*haqq*)."