Numbers 35:32-33

35:32 And you must not accept a ransom for anyone who has fled to a town of refuge, to allow him to return home and live on his own land before the death of the high priest.

35:33 “You must not pollute the land where you live, for blood defiles the land, and the land cannot be cleansed of the blood that is shed there, except by the blood of the person who shed it.