Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 45
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 45, with English translation by Maimonides (Rambam).
שלא להתגודד כעובדי עבודה זרה, שנאמר "לא תתגודדו" (דברים יד,א); וגדידה ושריטה אחת היא.
Not to make cuts in our flesh, as the worshipers of false gods do, as [Deuteronomy 14:1] states: "Do not cut yourselves." Gedidah is another term for cutting.