Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 357
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 357, with English translation by Maimonides (Rambam).
שלא תינשא היבמה לאחר חוץ מיבמה, שנאמר "לא תהיה אשת המת" (דברים כה,ה).
For a yevamah not to marry anyone other than her yavam, as [Deuteronomy 25:5] states: "The wife of the deceased shall not...."