Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 213
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 213, with English translation by Maimonides (Rambam).
לבעול בקידושין, שנאמר: "כי יקח איש אשה ובא אליה" (דברים כב, יג) וראה דברים כד, א.
To have sexual relations [only within] marriage, as [Deuteronomy 24:1] states: "If a man takes a woman [as a wife]...."