Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 190
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 190, with English translation by Maimonides (Rambam).
לעשות במלחמת הרשות כמשפט הכתוב בתורה, שנאמר: "כי תקרב אל עיר" (דברים כ,י).
To wage a voluntary war according to the laws prescribed by the Torah, as [Deuteronomy 20:10] states: "When you approach the city...."