Positive commandment · Mitzvat Aseh #225 of 613
To exile a person who accidentally kills a person
להגלות רוצח בשגגה
The verse · Bamidbar 35:25
וְהִצִּילוּ הָעֵדָה אֶת־הָרֹצֵחַ מִיַּד גֹּאֵל הַדָּם וְהֵשִׁיבוּ אֹתוֹ הָעֵדָה אֶל־עִיר מִקְלָטוֹ אֲשֶׁר־נָס שָׁמָּה וְיָשַׁב בָּהּ עַד־מוֹת הַכֹּהֵן הַגָּדֹל אֲשֶׁר־מָשַׁח אֹתוֹ בְּשֶׁמֶן הַקֹּדֶשׁ
“and the congregation shall deliver the slayer out of the hand of the revenger of blood, and the congregation shall restore him to the city of his refuge, whither he was fled: and he shall abide in it until the death of the high priest, who was anointed with the holy oil.”
The Rambam’s words
To exile a person who accidentally kills a person, as [Numbers 35:25] states: "And he shall dwell there until the High Priest dies...."
להגלות רוצח בשגגה, שנאמר: "וישב בה, עד מות הכוהן" (במדבר לה,כה).
Why · Sefer HaChinuch #410
The commandment on the court to send one who smites a soul inadvertently from his city to the cities of refuge and upon the killer himself to go there
“It is from the roots of the commandment [that] since the iniquity of killing is very weighty, as the destruction of the world [comes] through it, to the point that they, may their memory be blessed, said (Mishneh Torah, Laws of Murderer and the Preservation of Life 4:9) that one who kills a soul volitionally — even if he did all of the [other] commandments — is not saved from judgment, as it is stated (Proverbs 28:17), “A man oppressed by bloodguilt will flee to a pit; let none give him support”; therefore it is fitting for one who killed, that since such a great mishap as this came through his hand, that he should endure the pain of exile for it, which is almost equal to the pain of death — as a person is separated from his friends, and from his birthplace and [instead] dwells with strangers all of his days.”
Where in the Mishneh Torah
Hilchot Murderer and the Preservation of Life הלכות רוצח ושמירת נפש