Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 163
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 163, with English translation by Maimonides (Rambam).
שלא ייכנס כוהן למקדש פרוע ראש, שנאמר "ראשיכם אל תפרעו" (ויקרא י,ו).
For a priest not to enter the Temple with hair that has grown unseemingly long, as [implied by Leviticus 10:6]: "Do not let your hair grow long."