The Mishnah discusses the cities of refuge and the practical details of how an inadvertent killer is exiled there. It serves as the foundational rabbinic treatment of the halakhic structure of arei miklat.
נִגְמַר דִּינוֹ בְלֹא כֹהֵן גָּדוֹל, הַהוֹרֵג כֹּהֵן גָּדוֹל, וְכֹהֵן גָּדוֹל שֶׁהָרַג, אֵינוֹ יוֹצֵא מִשָּׁם לְעוֹלָם. וְאֵינוֹ יוֹצֵא לֹא לְעֵדוּת מִצְוָה וְלֹא לְעֵדוּת מָמוֹן וְלֹא לְעֵדוּת נְפָשׁוֹת. וַאֲפִלּוּ יִשְׂרָאֵל צְרִיכִים לוֹ, וַאֲפִלּוּ שַׂר צְבָא יִשְׂרָאֵל כְּיוֹאָב בֶּן צְרוּיָה, אֵינוֹ יוֹצֵא מִשָּׁם לְעוֹלָם, שֶׁנֶּאֱמַר (במדבר לה) אֲשֶׁר נָס שָׁמָּה, שָׁם תְּהֵא דִירָתוֹ, שָׁם תְּהֵא מִיתָתוֹ, שָׁם תְּהֵא קְבוּרָתוֹ. כְּשֵׁם שֶׁהָעִיר קוֹלֶטֶת, כָּךְ תְּחוּמָהּ קוֹלֵט. רוֹצֵחַ שֶׁיָּצָא חוּץ לַתְּחוּם וּמְצָאוֹ גוֹאֵל הַדָּם, רַבִּי יוֹסֵי הַגְּלִילִי אוֹמֵר, מִצְוָה בְיַד גּוֹאֵל הַדָּם, וּרְשׁוּת בְּיַד כָּל אָדָם. רַבִּי עֲקִיבָא אוֹמֵר, רְשׁוּת בְּיַד גּוֹאֵל הַדָּם, וְכָל אָדָם אֵין חַיָּבִין עָלָיו. אִילָן שֶׁהוּא עוֹמֵד בְּתוֹךְ הַתְּחוּם וְנוֹפוֹ נוֹטֶה חוּץ לַתְּחוּם, אוֹ עוֹמֵד חוּץ לַתְּחוּם וְנוֹפוֹ נוֹטֶה לְתוֹךְ הַתְּחוּם, הַכֹּל הוֹלֵךְ אַחַר הַנּוֹף. הָרַג בְּאוֹתָהּ הָעִיר, גּוֹלֶה מִשְּׁכוּנָה לִשְׁכוּנָה. וּבֶן לֵוִי, גוֹלֶה מֵעִיר לְעִיר:
If the verdict of a murderer was decided at a time when there was no High Priest, and likewise in the cases of one who unintentionally killed a High Priest and in the case of a High Priest who killed unintentionally, the unintentional murderer never leaves the city of refuge. And one who is exiled may not leave the city at all, either for testimony relating to a mitzva, or for testimony relating to monetary matters, or for testimony relating to capital matters. And even if the Jewish people require his services, and even if he is the general of the army of Israel like Joab ben Zeruiah, he never leaves the city of refuge, as it is stated: “And the congregation shall restore him to his city of refuge, that he fled there” (Numbers 35:25), from which it is derived: There shall be his dwelling, there shall be his death, there shall be his burial. The mishna continues: Just as an unintentional murderer is admitted to the city of refuge, so is he admitted to its outskirts, located within the Shabbat boundary. Once he entered the outskirts of the city, the blood redeemer may not kill him. In a case where a murderer emerged beyond the Shabbat boundary of the city of refuge and the blood redeemer found him there, Rabbi Yosei HaGelili says: It is a mitzva for the blood redeemer to kill him, and it is optional for every other person to do so. Rabbi Akiva says: It is optional for the blood redeemer, and every other person is liable for killing him. The previous mishna teaches that the halakhic status of the outskirts of the city is like that of the city itself in terms of the unintentional murderer being provided refuge there. The mishna adds: With regard to a tree that stands within the Shabbat boundary of a city of refuge, whose boughs extend outside the boundary, or a tree that stands outside the boundary and its boughs extend inside the boundary, the status of the tree, whether it is considered inside or outside the boundary, in all cases follows the boughs. If an unintentional murderer, exiled to a city of refuge, unintentionally killed a person in the same city, he is exiled from that neighborhood where he resided to another neighborhood within that city. And a Levite who is a permanent resident of a city of refuge and unintentionally killed a person is exiled from that city to another city.