And from where is "Aleph" called one, it is said, "How shall one rout one thousand?" And from where is the Holy One, blessed be He, called one as it is said "Hear O Israel the Lord our G-d the Lord is One". And from where is the Torah called one, as it is said, "One Torah will be for you." I am the head of all the pronouncements and "Aleph" is the head of all the letters:...
Why the First Hebrew Letter Aleph Is Called One
Yalkut Shimoni on Torah 2:1
וּמִנַּיִן שֶׁאָלֶ"ף נִקְרָא אֶחָד, שֶׁנֶּאֱמַר (דברים לב, ל) "אֵיכָה יִרְדֹּף אֶחָד אֶלֶף". וּמִנַּיִן שֶׁהַקָּדוֹשׁ בָּרוּךְ הוּא נִקְרָא אֶחָד, שֶׁנֶּאֱמַר (דברים ו, ד) "שְׁמַע יִשְׂרָאֵל ה' אֱלֹהֵינוּ ה' אֶחָד". וּמִנַּיִן שֶׁהַתּוֹרָה נִקְרֵאת אַחַת, שֶׁנֶּאֱמַר (במדבר טו, כט) "תּוֹרָה אַחַת יִהְיֶה לָכֶם" "אָנֹכִי" רֹאשׁ כָּל הַדִּבְּרוֹת, וְאָלֶ"ף רֹאשׁ כָּל הָאוֹתִיּוֹת.