The Messenger of Allah ﷺ said, "Whoever kills a person under a covenant of protection without just cause, Allah will forbid Paradise to him."Abu Abdul-Rahman said, "Kunhihi means his rightful due."
حدثنا وكيع وأبو عبد الرحمن قالا حدثنا عيينة عن أبيه
قال رسول الله ﷺ من قتل معاهدا في غير كنهه حرم الله عليه الجنة قال أبو عبد الرحمن كنهه حق
Its isnad is sahih. It is found in Abu Dawud 3/84, no. 2760, in jihad, chapter on fulfilling covenants with a protected non-Muslim, and in Nasai 8/34, in qasama, chapter on the severity of killing a protected non-Muslim, and in Darimi in siyar 2/235. Hakim graded it sahih 2/142, and Dhahabi agreed with him.