Bamidbar 35 discusses the cities that are to be given to the Leviem, and says that in addition to the 6 cities of refuge they are to be given 42 cities.
וְאֵ֣ת הֶֽעָרִ֗ים אֲשֶׁ֤ר תִּתְּנוּ֙ לַלְוִיִּ֔ם אֵ֚ת שֵׁשׁ־עָרֵ֣י הַמִּקְלָ֔ט אֲשֶׁ֣ר תִּתְּנ֔וּ לָנֻ֥ס שָׁ֖מָּה הָרֹצֵ֑חַ וַעֲלֵיהֶ֣ם תִּתְּנ֔וּ אַרְבָּעִ֥ים וּשְׁתַּ֖יִם עִֽיר׃ The towns that you assign to the Levites shall comprise the six cities of refuge that you are to designate for a manslayer to flee to, to which you shall add forty-two towns.
The stam, and then Abaye in BT Makos take for granted that these 42 additional cities also serve as cities of refuge.
ותו ליכא והא כתיב (במדבר לה, ו) ועליהם תתנו ארבעים ושתים עיר אמר אביי הללו קולטות בין לדעת בין שלא לדעת הללו לדעת קולטות שלא לדעת אינן קולטות The Gemara asks: And are there no more cities of refuge beyond these six? But isn’t it written: “And the cities that you shall give to the Levites: The six cities of refuge you shall give for the murderer to flee there, and beyond them you shall give forty-two cities” (Numbers 35:6), indicating that the status of all the Levite cities is that of cities of refuge? Abaye said: With regard to these six cities designated specifically for this purpose, unintentional murderers in need of refuge are admitted there whether they entered the cities deliberately, aware that they are cities of refuge, or whether they entered inadvertently. By contrast, with regard to those forty-two Levite cities, unintentional murderers are admitted only if they entered the cities deliberately, but if they entered the cities inadvertently, they are not admitted to the cities.
However I could locate not any prior source that hinted at such a reading of the pesukim. I am looking for any biblical or taanitic source that might indicate such an understanding. (looked at Bamidbar 35, Devarim 19, Yehoshua 20, Mishna, Tosefta, Midrash, Yerushalmi makos.) Although the Talmud's question above quotes ועליהם תתנו ארבעים ושתים עיר that pasuk does not show that those 42 have the same function-- the section is discussion the territory to be given to the Leveim. Answers that distinguish drash from pshat here should please explain a compelling reason to have the drash diverge in this way. I am more interested in precedent than the drasha itself as the Talmud takes it as a given rather than asking it as a contradiction between some other source that makes such a drasha, indicating that it was accepted and clear that the 42 serve as cities of refuge.