Midrash for Makkot 18:23

(Ibid. 3) "Prepare for yourself the way": Make stations leading into them.

(Devarim 19:11) "and he rise up against him, etc." (Ibid. 12) "Then the elders of his city shall send, and they shall take him from there": From here R. Yossi b. Yehudah says: If one slays a soul, whether unwittingly or wittingly — all proceed to the cities of refuge, and beth-din send and bring him from there.