The Messenger of Allah ﷺ said, "Whoever kills a person under covenant without just cause, Allah has forbidden Paradise to him."
أخبرنا إسمعيل بن مسعود قال حدثنا خالد عن عيينة قال أخبرني أبي قال
قال رسول الله ﷺ من قتل معاهدا في غير كنهه حرم الله عليه الجنة
[Its chain of transmission is sahih] Abu Dawud narrated it in the chapter on jihad, in the section on fulfilling covenants with treaty partners and the sanctity of their protection, no. 2760, from the hadith of Uyayna bin Abdul-Rahman bin Jawshan, transmitted through him. It is also found in Al-Kubra, no. 6949. Ibn Hibban, Ibn Jarud, and others graded it sahih.