Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 314
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 314, with English translation by Maimonides (Rambam).
שלא לגרוע מכל מצוות התורה, שנאמר "ולא תגרע ממנו" (דברים יג,א).
Not to diminish from any of the mitzvot of the Torah, as [Deuteronomy, ibid.] continues: "...and do not diminish from it."