Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 42
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 42, with English translation by Maimonides (Rambam).
שלא ללבוש שעטנז כמו שלובשין כומרי עבודה זרה, שנאמר "לא תלבש שעטנז" (דברים כב,יא).
Not to wear sha'atnez, as do the priests of false gods, as [Deuteronomy 22:11] states: "Do not wear sha'atnez."