The Talmud (Baba Metziah 61b) quotes the following question and answer posed by Rava. Why does the Torah mention the Exodus from Egypt alongside the following commandments:
Interest (“Do not lend him money at advance interest, or give him your food at increase. I am the Lord your G-d who took you out of Egypt…” Leviticus 25:37-38)
Tzitzit (“And it shall be for them tzitzit…I am the Lord your G-d who took you out of Egypt…” Numbers 15:39-41)
Measures (“You shall not falsify measures of length, weight, or capacity. You shall have an honest balance, honest weights…I am the Lord your G-d who took you out of Egypt.” Numbers 19:35-36)
Rava replies:
The Holy One Blessed Be He said, “I am the One who distinguished in Egypt between the drop [which created] a firstborn and the drop of a non-firstborn. I am the One who will exact retribution from one who associates his money with a non-Jew and lends it to a Jew with interest. And from one who tilts his weights with salt. And from one who wears kala ilan on his cloak and says it is tekhelet.”
Rav Menahem Borstein, in his book HaTekhelet, explains this phenomenom. The great expense of acquiring and/or producing tekhelet from the chilazon brought about attempts to counterfeit the commodity. Dye matter of plant origin (“kala ilan”) was more readily and less expensively processed, and could successfully produce a color identical to tekhelet, to the extent that only G-d could tell them apart. The counterfeit production was so well-known that recipes for preparing “fake-tekhelet” were recorded (and uncovered) on papyrus!
