רֵאשִׁ֗ית בִּכּוּרֵי֙ אַדְמָ֣תְךָ֔ תָּבִ֕יא בֵּ֖ית יְהוָ֣ה אֱלֹהֶ֑יךָ לֹֽא־תְבַשֵּׁ֥ל גְּדִ֖י בַּחֲלֵ֥ב אִמּֽוֹ׃ (ס)
The choicest first-fruits of thy land thou shalt bring into the house of the LORD thy God. Thou shalt not seethe a kid in its mother’s milk.
Read kabbalistic (mystical) interpretation of Exodus 23:19: and classical mystical sources.