Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 219
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 219, with English translation by Maimonides (Rambam).
שלא לחסום בהמה בשעת מלאכה בדבר שאוכלת ממנו ונהנית, שנאמר "לא תחסום שור, בדישו" (דברים כה,ד).
Not to muzzle an ox while it is working with produce from which it would eat and derive benefit, as [Deuteronomy 25:4] states: "Do not muzzle an ox while it is treading grain."