Torah Portion: Mishpatim
He may not diminish her food, her clothing, or her conjugal rights (Shemos 21:10).
It is a negative commandment that whoever purchases a Hebrew maidservant and designated her for marriage, is not to leave her short of her rightful sh’er [food/allowance], k’sus [clothing] and ’onah [conjugal rights]. This commandment extends to one who marries any Jewish woman—that none of these things is to be diminished for them. This is derived through a kal va-chomer (a fortiori) reasoning [from the lesser to the greater]: If for this Hebrew maidservant one may not diminish any of these, how much more so for others, who are free women.
This mitzvah applies with regard to a free daughter [i.e., a regular Jewish woman] in all places and at all times, to men. One who transgresses it and, of his free will, leaves his wife short of any of these three, and intended to make her suffer, violates a negative prohibition.