Negative commandment · Lo Ta’aseh #355 of 613
Not to have sexual relations with a woman without a marriage contract and betrothal
שלא תהיה קדשה, והיא הנבעלת בלא כתובה וקידושין
The verse · Devarim 23:18
לֹא־תִהְיֶה קְדֵשָׁה מִבְּנוֹת יִשְׂרָאֵל וְלֹא־יִהְיֶה קָדֵשׁ מִבְּנֵי יִשְׂרָאֵל
“There shall be no female prostitute of the daughters of Yisra᾽el, nor a male prostitute of the sons of Yisra᾽el.”
The Rambam’s words
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...."
שלא תהיה קדשה, והיא הנבעלת בלא כתובה וקידושין, שנאמר "לא תהיה קדשה" (דברים כג,יח).
Who & when
“And this prohibition is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Marriage הלכות אישות