The 225th mitzvah is that we are commanded to exile a person who has killed another unintentionally from his city to one of the refuge cities.

The source of this commandment is G‑d's statement1 (exalted be He), "[The killer] must live there until the death of the kohen gadol."

In the words of the Sifri, "In the phrase '[The killer] must live there,' the word 'there' means he may never leave there. There he shall live, there he shall die, and there he shall be buried."

The details of this mitzvah are explained in tractate Makkos.2