Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 215
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 215, with English translation by Maimonides (Rambam).
למול את הבן, שנאמר: "וביום השמיני ימול בשר ערלתו" (ויקרא יב, ג).
To circumcise a son, as [Leviticus 12:3] states: "On the eighth day, the flesh of his foreskin shall be circumcised." 7:6.)