Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 209
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 209, with English translation by Maimonides (Rambam).
שלא יגלח הנזיר, שנאמר "תער לא יעבור על ראשו" (במדבר ו,ה).
[For a Nazarite] not to shave, as [Numbers 6:5] states: "A razor shall not pass upon his head."