Whoever kills a mu‘āhad (a non-Muslim who is granted the pledge of protection by the Muslims) shall not smell the fragrance of Paradise, though its fragrance can be smelt at a distance of forty years

It is prohibited to kill a mu‘āhad, a dhimmi, or a musta’man. This is a major sin.
Mu‘āhad: He is a disbeliever with whom a covenant was made while he lives in his land and does not fight the Muslims, nor do they fight him. Dhimmi: He is a non-Muslim who comes to live in the Muslim land and pays tribute. Musta’man: He is a non-Muslim who enters the land of the Muslims under a covenant and pledge of protection for a certain period of time.
It warns against the betrayal of covenants with non-Muslims.