Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 296
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 296, with English translation by Maimonides (Rambam).
שלא ליקח כופר בגלות רוצח בשגגה, שנאמר "וְלֹא תִקְחוּ כֹּפֶר לָנוּס אֶל עִיר מִקְלָטוֹ" (במדבר לה, לב).
Not to take a ransom in return for exile for a person who kills accidentally, as [Numbers 35:32] states: "And do not take a ransom [for having] to flee to his refuge city."