Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 355
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 355, with English translation by Maimonides (Rambam).
שלא תהיה קדשה, והיא הנבעלת בלא כתובה וקידושין, שנאמר "לא תהיה קדשה" (דברים כג,יח).
For there not to be a kedeishah - i.e., a woman who engages in intimate relations without a marriage contract and a marriage ceremony - [among the Jewish people], as [Deuteronomy 23:18] states: "There shall not be a kedeishah...."