Tosefta for Berakhot 4:7
רַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה אוֹמֵר, אֵין תְּפִלַּת הַמּוּסָפִין אֶלָּא בְּחֶבֶר עִיר. וַחֲכָמִים אוֹמְרִים, בְּחֶבֶר עִיר וְשֶׁלֹּא בְחֶבֶר עִיר. רַבִּי יְהוּדָה אוֹמֵר מִשְּׁמוֹ, כָּל מָקוֹם שֶׁיֵּשׁ חֶבֶר עִיר, הַיָּחִיד פָּטוּר מִתְּפִלַּת הַמּוּסָפִין:
R. Elazar b. Azaryah says: The mussaf ("additional") prayer is recited only bechever ir [bechavurath ha'ir ("with a company of the city"); that is, with a congregation, and not individually.] And the sages say: Bechever ir and not bechever ir [both with a congregation and individually.] R. Yehudah says in his [R. Elazar's] name: Wherever there is a congregation, an individual is exempt from the recitation of the mussaf prayer. [The difference between the first tanna and R. Yehudah — an individual living in a city where there are not ten men. According to the first tanna citing R. Elazar to the effect that it was instituted only with a congregation, this individual is exempt. According to R. Yehudah, an individual is exempt only when he is in a place where there are ten men, in which instance the prayer leader exempts him. The halachah is according to the sages.]