Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 216
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 216, with English translation by Maimonides (Rambam).
לייבם אשת אח, שנאמר: "יבמה יבוא עליה" (דברים כה,ה).
To marry the widow of one's brother who died childless, as [Deuteronomy 25:5] states: "Her yavam shall come unto her."