Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 359
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 359, with English translation by Maimonides (Rambam).
שלא יגרש מוציא שם רע את אשתו, שנאמר בו "לא יוכל לשלחה כל ימיו" (דברים כב,יט).
For a person who defamed his wife's character not to divorce her, as [Deuteronomy 22:19] states: "He may not send her away for his entire life."