Joshua 20:3

Anyone who kills someone accidentally and unintentionally may go there for a refuge from the avenger of blood.

Treasury of Scripture Knowledge

Summary

That the slayer that killeth any person unawares and unwittingly may flee thither: and they shall be your refuge from the avenger of blood.

General references

Bible References

General references

Numbers 35:19
The relative who can avenge the death must make sure a murderer is put to death. When he catches up with the murderer, he must kill him.