Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 52
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 52, with English translation by Maimonides (Rambam).
שלא להתחתן בעובדי עבודה זרה, שנאמר "ולא תתחתן, בם" (דברים ז,ג).
Not to marry gentiles, as [Deuteronomy 7:3] states: "Do not marry among them."