Behold to you
In the question: Where does it come from? See for yourself. That is how things are.
("The Holy One, blessed be He, does not give greatness to a man until He tests him in a small matter and then elevates him to greatness. Blessed are you, two great men of the world, whom the Holy One, blessed be He, tested in a small matter and found faithful, and exalted to greatness...in David's case...and so in Moses" - Shemot Rava (Vilna) Parashat Shemot Parshat 2)
Translation
Expression (Hebrew)
הֲרֵי לְךָ
Definition (Hebrew)
בְּהַשְׁאָלָה: מִכָּאן נוֹבֵעַ. רְאֵה בְּעַצְמְךָ. כָּךְ הֵם פְּנֵי הַדְּבָרִים.
("אֵין הקב"ה נוֹתֵן גְּדֻלָּה לְאָדָם עַד שֶׁבּוֹדְקֵהוּ בְּדָבָר קָטָן ואח"כ מַעֲלֵהוּ לִגְדֻלָּה, הֲרֵי לְךָ שְׁנֵי גְּדוֹלֵי עוֹלָם שֶׁבְּדֵקָן הקב"ה בְּדָבָר קָטָן וְנִמְצְאוּ נֶאֱמָנִים וְהֶעֱלָן לִגְדֻלָּה...בָּדַק לְדָוִד ... וְכֵן בְּמֹשֶׁה"- שְׁמוֹת רַבָּה (וִילְנָא) פָּרָשַׁת שְׁמוֹת פָּרָשָׁה ב)