Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 38
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 38, with English translation by Maimonides (Rambam).
להיות כהן גדול נושא בתולה, שנאמר: "וְהוּא אִשָּׁה בִּבְתוּלֶיהָ יִקָּח" (ויקרא כא, יג).
For a High Priest to marry a virgin, as [Leviticus 21:13] states: "And he shall marry a woman who is a virgin."