"Go down there": whoever buys produce from the market is brought low. (Genesis 42:5) From where do we learn that an individual may not recite "Holy"? As it is said, "and I shall be sanctified among the children of Israel," and it is written elsewhere (Numbers 16:21) "separate yourselves from the midst of this congregation"—just as there it refers to ten, so here it refers to ten. Rabbi Yose bar Avin said: if you derive it from the word "midst," they would be many. Rather, it says here "the children of Israel" (Leviticus 22:32), and it says elsewhere "and the children of Israel came to buy [grain]"—just as there it means ten, so here it means ten.
Ten for a Quorum Learned From Joseph's Brothers
Yalkut Shimoni on Torah 148:6
רְדוּ שָׁמָּה, כָּל הַלּוֹקֵחַ תְּבוּאָה מִן הַשּׁוּק יָרוּד. (בראשית מב ה) מִנַּיִן שֶׁאֵין הַיָּחִיד אוֹמֵר קָדוֹשׁ, שֶׁנֶּאֱמַר "וְנִקְדַּשְׁתִּי בְּתוֹךְ בְּנֵי יִשְׂרָאֵל" וּכְתִיב הָתָם (במדבר טז, כא) "הִבָּדְלוּ מִתּוֹךְ הָעֵדָה הַזֹּאת" מָה לְהַלָּן עֲשָׂרָה אַף כַּאן עֲשָׂרָה. אָמַר רַבִּי יוֹסֵי בַּר אָבִין, אִם מִתּוֹךְ אַתְּ יָלִיף סַגִּיאִין אִנּוּן אֶלָּא נֶאֱמַר (ויקרא כב, לב) "בְּנֵי יִשְׂרָאֵל" וְנֶאֱמַר לְהַלָּן וַיָּבֹאוּ בְּנֵי יִשְׂרָאֵל לִשְׁבֹּר (בַּר), מָה לְהַלָּן עֲשָׂרָה אַף כָּאן עֲשָׂרָה.