Negative commandment · Lo Ta’aseh #215 of 613
Not to sow mixed species of seeds together
שלא לזרוע כלאי זרעים
The verse · Vayikra 19:19
אֶת־חֻקֹּתַי תִּשְׁמֹרוּ בְּהֶמְתְּךָ לֹא־תַרְבִּיעַ כִּלְאַיִם שָׂדְךָ לֹא־תִזְרַע כִּלְאָיִם וּבֶגֶד כִּלְאַיִם שַׁעַטְנֵז לֹא יַעֲלֶה עָלֶיךָ
“You shall keep my statutes. Thou shalt not let thy cattle gender with a diverse kind: thou shalt not sow thy field with mingled seed: neither shall a garment mingled of linen and wool come upon thee.”
The Rambam’s words
Not to sow mixed species of seeds together, as [Leviticus 19:19] states: "Do not sow different species of seed in your field."
שלא לזרוע כלאי זרעים, שנאמר "שדך לא תזרע כלאיים" (ויקרא יט,יט).
Why · Sefer HaChinuch #245
To not sow seeds of forbidden mixtures and not graft in any place in the Land
“From the roots of the commandment are that it is known that magic is a very bad thing and causes many mishaps to people. I do not have to write at length about it, as the things are well-known.”
Who & when
“And the prohibition of forbidden mixtures of seeds is practiced by males and females only in the Land of Israel, as we said.”
Where in the Mishneh Torah
Hilchot Diverse Species הלכות כלאים