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I can’t find an English translation, but it’s Leviticus 19:19. It’s also later in Deuteronomy 22:9-12.

Here is my pathetic attempt at translation:

“You must obey My Law. You must not let your animals breed with animals of a different kind. You must not plant your field with two kinds of seed. You must not wear clothing made from two kinds of material mixed together.”

The forbidden idea of mixing species (or things, like fabrics) is called “Kilayim” in Judaism.

It’s not an obscure idea; looking at my index, there are 70-80 mishnahs on the topic of cross-breeding.

Maimondes in his book is very specific what this meant:

When a person causes a male to enter into relations with a female of a different species - whether a domesticated animal, a wild beast, a fowl, even different types of sea-animals - he is liable for lashes according to Scriptural Law. [This applies] in all places in Eretz Yisrael and in the Diaspora, as [Leviticus 19:19] states: “You shall not mate your animal with another species.”

This applies whether the animal, beast, or fowl belongs to him or to a colleague.

(from Maimondes — Kilaayim - Chapter 9

HaShem’s reasoning is not known. Note the “You must obey My Laws” bit at the beginning.

The word for “statutes” in the original Hebrew is “chukim” (חקים), which usually refers to those laws which have no known reason.

This basically translates to “because I said so.” Any parent knows why you say that. The kid is not ready for the discussion.

Um? Maybe because you are Bronze Age morons and haven’t figure out DNA?

Nevertheless, Nachmanides took a stab at it:

G‑d instilled in the various species the ability to reproduce so that that species will continue to exist for as long as He wishes. When one crossbreeds (whether animals or plants), it is as if the person is indicating that G‑d didn’t create all necessary beings, and this person wants to assist in the creation of the world.

Additionally, crossbreeding means that the species that was meant to be continued through the power of reproduction is now being (somewhat) discontinued, and instead producing a different species.