Negative commandment · Lo Ta’aseh #262 of 613
Not to reduce her flesh, covering and time period
שלא למנוע מאמה עברייה היעודה שאר כסות ועונה
The verse · Shemot 21:10
אִם־אַחֶרֶת יִקַּח־לוֹ שְׁאֵרָהּ כְּסוּתָהּ וְעֹנָתָהּ לֹא יִגְרָע
“If he take another wife for himself; her food, her clothing, and her duty of marriage, shall he not diminish.”
The Rambam’s words
Not to withhold from a Hebrew maidservant who has been designated as a bride living expenses, clothing, or conjugal rights, as [Exodus 21:10] states: "He may not diminish her living expenses, clothing, or conjugal rights." The above applies also to other wives.
שלא למנוע מאמה עברייה היעודה שאר כסות ועונה, שנאמר "שְׁאֵרָהּ כְּסוּתָהּ וְעוֹנָתָהּ לֹא יִגְרָע" (שמות כא, י); והוא הדין לשאר הנשים.
Who & when
“And this commandment is practiced regarding a free woman in every place and at all times by males.”
Where in the Mishneh Torah
Hilchot Marriage הלכות אישות