Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 361
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 361, with English translation by Maimonides (Rambam).
שלא לסרס זכר מכל המינין, לא אדם ולא בהמה חיה ועוף, שנאמר "ובארצכם, לא תעשו" (ויקרא כב,כד).
Not to castrate a male from any species - neither a human, a domestic animal, a wild beast, nor a bird, as [Leviticus 22:24] states: "You shall not do this in your land."