Negative commandment · Lo Ta’aseh #42 of 613
Not to wear sha'atnez, as do the priests of false gods
שלא ללבוש שעטנז כמו שלובשין כומרי עבודה זרה
The verse · Devarim 22:11
לֹא תִלְבַּשׁ שַׁעַטְנֵז צֶמֶר וּפִשְׁתִּים יַחְדָּו
“Thou shalt not wear a garment of divers kinds, of woollen and linen together.”
The Rambam’s words
Not to wear sha'atnez, as do the priests of false gods, as [Deuteronomy 22:11] states: "Do not wear sha'atnez."
שלא ללבוש שעטנז כמו שלובשין כומרי עבודה זרה, שנאמר "לא תלבש שעטנז" (דברים כב,יא).
Why · Sefer HaChinuch #551
To not wear shaatnez
“From the roots of the commandment are that it is known that magic is a very bad thing and causes many mishaps to people. I do not have to write at length about it, as the things are well-known.”
Who & when
“And this prohibition is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Diverse Species הלכות כלאים