Numbers 35:27 Midrash

וּמָצָ֤א אֹתוֹ֙ גֹּאֵ֣ל הַדָּ֔ם מִח֕וּץ לִגְב֖וּל עִ֣יר מִקְלָט֑וֹ וְרָצַ֞ח גֹּאֵ֤ל הַדָּם֙ אֶת־הָ֣רֹצֵ֔חַ אֵ֥ין ל֖וֹ דָּֽם׃

and the avenger of blood find him without the border of his city of refuge, and the avenger of blood slay the manslayer; there shall be no bloodguiltiness for him;