Negative commandment · Lo Ta’aseh #348 of 613
Not to perform a sexual act with an animal
שלא לשכב עם בהמה
The verse · Vayikra 18:23
וּבְכׇל־בְּהֵמָה לֹא־תִתֵּן שְׁכׇבְתְּךָ לְטׇמְאָה־בָהּ וְאִשָּׁה לֹא־תַעֲמֹד לִפְנֵי בְהֵמָה לְרִבְעָהּ תֶּבֶל הוּא
“Neither shalt thou lie with any beast to defile thyself with it: neither shall any woman stand before a beast to lie down before it: it is perversion.”
The Rambam’s words
Not to perform a sexual act with an animal, as [Leviticus 18:23] states: "And do not lie carnally with any animal."
שלא לשכב עם בהמה, שנאמר "ובכל בהמה לא תיתן שכובתך, לזרע" (ראה ויקרא יח,כ; ויקרא יח,כג).
Why · Sefer HaChinuch #210
To not lay with a beast
“It is from the roots of the commandment that G-d, blessed be He, desired that all the species that He created in His world make offspring according to their species and like it appears in Scripture at the beginning of the Creation, such that it is written with each and every species, “according to its species.””
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה