TT #192 · Parashas Shoftim · Fifth Aliyah · Passuk 19:02
You shall set aside three cities for yourself in the midst of your land that the Eternal your God is giving you to possess.
Torah Temimah points to Sifrei Devarim 180 / Makkos.
TT lemma: שָׁלוֹשׁ עָרִים תַּבְדִּיל.
Sefaria — Sifrei Devarim 180 / Makkos
English: ‘You shall set aside three cities for yourself’ – in the land of Canaan.
Derash on the location (Sifrei / Bavli) — the verse establishes the first set of cities of refuge in the Land proper.
Three cities of refuge must be designated in the Land of Canaan for the accidental killer.