Rashi on Deuteronomy 26:17

האמרת … האמירך. אֵין לָהֶם עֵד מוֹכִיחַ בַּמִּקְרָא, וְלִי נִרְאֶה שֶׁהוּא לְשׁוֹן הַפְרָשָׁה וְהַבְדָּלָה — הִבְדַּלְתָּ לְךָ מֵאֱלֹהֵי הַנֵּכָר לִהְיוֹת לְךָ לֵאלֹהִים וְהוּא הִפְרִישְׁךָ אֵלָיו מֵעַמֵּי הָאָרֶץ לִהְיוֹת לוֹ לְעַם סְגֻלָּה, וּמָצָאתִי לָהֶם עֵד וְהוּא לְשׁוֹן תִּפְאֶרֶת כְּמוֹ (תהלים צ"ד) "יִתְאַמְּרוּ כָּל פֹּעֲלֵי אָוֶן": האמרה, האמירך are words for the meaning of which there is no decisive proof in Scripture. It seems to me, however, that they are expressions denoting “separation” and “selection”: “You have singled Him out from all strange gods to be unto you as God — and He on His part, has singled you out from the nations on earth to be unto Him a select people”. [As far as this meaning is concerned I have found a parallel (lit., a witness) to it where it bears the meaning “glory”, as in (Psalms 94:4): “All wrongdoers glory in themselves”].

כאשר דבר לך. וִהְיִיתֶם לִי סְגֻלָּה: כאשר דבר לך AS HE HATH PROMISED THEE — in the words (Exodus 19:5) “Ye shall be a select portion unto Me” (Mekhilta).