The stranger who resides with you shall be to you as one of your citizens; you shall love him as yourself, for you were strangers in the land of Egypt: I the LORD am your God. You shall not falsify measures of length, weight, or capacity. (36) You shall have an honest balance, honest weights, an honest ephah, and an honest hin. I the LORD am your God who freed you from the land of Egypt. You shall faithfully observe all My laws and all My rules: I am the LORD.
אשר הוצאתי אתכם WHO BROUGHT YOU FORTH [FROM EGYPT] for this purpose, that you shall be honest in your actions (Sifra, Kedoshim, Chapter 8 10).
Another explanation is: I, God, distinguished in Egypt between those who were firstborn and those who were not firstborn, I am also certain to exact punishment from the one who secretly puts his weights in salt in order to take advantage of people who are not aware of it (cf. Bava Metzia 61b).
The punishment for faking measurements is greater than the punishment for forbidden sexual relationships, for one is a sin between a person and God, and the other has an interpersonal dimension. Rejecting the commandment of fair weights is like a rejection of the exodus from Egypt, which was the source of the commandment. Likewise, anyone who accepts the commandment of fair weights is like one who accepts the reality of the exodus from Egypt, which is the source of all commandments.
Anyone who falsifies measurements is covering up their behaviour, making sure nobody will see them, and doesn't fear being seen by the Holy One - such people think that there is no divine supervision over human actions. And anybody who rejects this principle is like one who rejects the Exodus, since at that time divine supervision over us was made most manifest, with all the miracles and signs in Egypt.
Now the LORD had said, “Shall I hide from Abraham what I am about to do, since Abraham is to become a great and populous nation and all the nations of the earth are to bless themselves by him? For I have singled him out, that he may instruct his children and his posterity to keep the way of the LORD by doing what is just and right, in order that the LORD may bring about for Abraham what He has promised him.”
(ה) רְאֵ֣ה ׀ לִמַּ֣דְתִּי אֶתְכֶ֗ם חֻקִּים֙ וּמִשְׁפָּטִ֔ים כַּאֲשֶׁ֥ר צִוַּ֖נִי ה' אֱלֹקָ֑י לַעֲשׂ֣וֹת כֵּ֔ן בְּקֶ֣רֶב הָאָ֔רֶץ אֲשֶׁ֥ר אַתֶּ֛ם בָּאִ֥ים שָׁ֖מָּה לְרִשְׁתָּֽהּ׃ (ו) וּשְׁמַרְתֶּם֮ וַעֲשִׂיתֶם֒ כִּ֣י הִ֤וא חָכְמַתְכֶם֙ וּבִ֣ינַתְכֶ֔ם לְעֵינֵ֖י הָעַמִּ֑ים אֲשֶׁ֣ר יִשְׁמְע֗וּן אֵ֚ת כָּל־הַחֻקִּ֣ים הָאֵ֔לֶּה וְאָמְר֗וּ רַ֚ק עַם־חָכָ֣ם וְנָב֔וֹן הַגּ֥וֹי הַגָּד֖וֹל הַזֶּֽה׃ (ז) כִּ֚י מִי־ג֣וֹי גָּד֔וֹל אֲשֶׁר־ל֥וֹ אֱלֹקִ֖ים קְרֹבִ֣ים אֵלָ֑יו כַּה' אֱלֹקֵ֔ינוּ בְּכָּל־קָרְאֵ֖נוּ אֵלָֽיו׃ (ח) וּמִי֙ גּ֣וֹי גָּד֔וֹל אֲשֶׁר־ל֛וֹ חֻקִּ֥ים וּמִשְׁפָּטִ֖ים צַדִּיקִ֑ם כְּכֹל֙ הַתּוֹרָ֣ה הַזֹּ֔את אֲשֶׁ֧ר אָנֹכִ֛י נֹתֵ֥ן לִפְנֵיכֶ֖ם הַיּֽוֹם׃ (ט) רַ֡ק הִשָּׁ֣מֶר לְךָ֩ וּשְׁמֹ֨ר נַפְשְׁךָ֜ מְאֹ֗ד פֶּן־תִּשְׁכַּ֨ח אֶת־הַדְּבָרִ֜ים אֲשֶׁר־רָא֣וּ עֵינֶ֗יךָ וּפֶן־יָס֙וּרוּ֙ מִלְּבָ֣בְךָ֔ כֹּ֖ל יְמֵ֣י חַיֶּ֑יךָ וְהוֹדַעְתָּ֥ם לְבָנֶ֖יךָ וְלִבְנֵ֥י בָנֶֽיךָ׃...׃ (כ) וְאֶתְכֶם֙ לָקַ֣ח ה' וַיּוֹצִ֥א אֶתְכֶ֛ם מִכּ֥וּר הַבַּרְזֶ֖ל מִמִּצְרָ֑יִם לִהְי֥וֹת ל֛וֹ לְעַ֥ם נַחֲלָ֖ה כַּיּ֥וֹם הַזֶּֽה׃
See, I have imparted to you laws and rules, as the LORD my God has commanded me, for you to abide by in the land that you are about to enter and occupy. Observe them faithfully, for that will be proof of your wisdom and discernment to other peoples, who on hearing of all these laws will say, “Surely, that great nation is a wise and discerning people.” For what great nation is there that has a god so close at hand as is the LORD our God whenever we call upon Him? Or what great nation has laws and rules as perfect as all this Teaching that I set before you this day?
But take utmost care and watch yourselves scrupulously, so that you do not forget the things that you saw with your own eyes and so that they do not fade from your mind as long as you live. And make them known to your children and to your children’s children... but you the LORD took and brought out of Egypt, that iron blast furnace, to be His very own people, as is now the case.
"Blessed is the Holy One, Blessed is They, Blessed is the One who gave us Torah." One should ponder on the relevance here of "giving the Torah". It appears to me, however, that the letters of the Torah were also in exile in Egypt. And it was on account of the iron furnace that the exiles were able to leave their exile and come to the time of the giving of the Torah. This is what is meant by (Exodus 3:12) "And this shall be the sign (ot)" - meaning the letters (otiot) of the Torah.
Now the 27 letters of the Torah are pure, (zach), and each of them consists of all of them. 27 multiplied by 27 is equal to "Pesach, Matzah and Maror." Therefore these are the three things that Rabban Gamliel obligated us to declare on Pesach, in order to show the greatness of using the mouth to express the letters of the Torah which we are commanded to speak.