Numbers 35:19

35:19 The avenger of blood himself must kill the murderer; when he meets him, he must kill him.

Numbers 35:26

35:26 But if the slayer at any time goes outside the boundary of the town to which he had fled,

Numbers 35:31

35:31 Moreover, you must not accept a ransom for the life of a murderer who is guilty of death; he must surely be put to death.