Numbers 35:21

35:21 or with enmity he strikes him with his hand and he dies, the one who struck him must surely be put to death, for he is a murderer. The avenger of blood must kill the murderer when he meets him.

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.