Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 225
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 225, with English translation by Maimonides (Rambam).
להגלות רוצח בשגגה, שנאמר: "וישב בה, עד מות הכוהן" (במדבר לה,כה).
To exile a person who accidentally kills a person, as [Numbers 35:25] states: "And he shall dwell there until the High Priest dies...."