Numbers 35:19-23

The revenger of blood himself shall slay the murderer: when he meeteth him, he shall slay him.

But if he thrust him of hatred, or hurl at him by laying of wait, that he die;

Or in enmity smite him with his hand, that he die: he that smote him shall surely be put to death; for he is a murderer: the revenger of blood shall slay the murderer, when he meeteth him.

But if he thrust him suddenly without enmity, or have cast upon him any thing without laying of wait,

Or with any stone, wherewith a man may die, seeing him not, and cast it upon him, that he die, and was not his enemy, neither sought his harm: