Negative commandment · Lo Ta’aseh #264 of 613
Not to force a yefat to'ar to serve as a maidservant
שלא לכבוש אשת יפת תואר שפחה
The verse · Devarim 21:14
וְהָיָה אִם־לֹא חָפַצְתָּ בָּהּ וְשִׁלַּחְתָּהּ לְנַפְשָׁהּ וּמָכֹר לֹא־תִמְכְּרֶנָּה בַּכָּסֶף לֹא־תִתְעַמֵּר בָּהּ תַּחַת אֲשֶׁר עִנִּיתָהּ
“And it shall be, if thou have no delight in her, then thou shalt let her go where she will; but thou shalt not sell her at all for money, thou shalt not treat her as a slave, because thou hast humbled her.”
The Rambam’s words
Not to force a yefat to'ar to serve as a maidservant, as [Deuteronomy, ibid.] continues: "Do not rule over her."
שלא לכבוש אשת יפת תואר שפחה, שנאמר "לֹא תִתְעַמֵּר בָּהּ" (דברים כא, יד).
Why · Sefer HaChinuch #534
Not to make her serve after he has intercourse with her
“It is from the roots of the commandment to teach ourselves good and precious traits. And I have already written in many places that a precious soul is fit to receive goods and upon it will blessing always descend, and that the good G-d desired to crown His people with every lovely and refined trait.”
Where in the Mishneh Torah
Hilchot Kings and Wars הלכות מלכים ומלחמות