Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 44
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 44, with English translation by Maimonides (Rambam).
שלא להשחית כל הזקן כעובדי עבודה זרה, שנאמר "ולא תשחית, את פאת זקנך" (ויקרא יט,כז).
Not to shave off our beards entirely, as do the priests of false gods, as [Leviticus 19:27] states: "Do not shave the corners of your beard."