Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 336

שלא לגלות ערוות הבת; ולמה לא נתפרשה בתורה, מפני שאסר בת הבת שתק מן הבת, ומפי השמועה למדו שאיסור הבת מגופי תורה, כשאר עריות.

Not to have intimate relations with one's daughter. Why was this prohibition not explicitly stated in the Torah? Since the Torah forbade [relations with] one's daughter's daughter, it did not mention [the prohibition against relations with] one's daughter. [Nevertheless, according to the oral tradition, the prohibition against [relations with] one's daughter has the status of a Torah law like the other sexual offences [and is not considered as Rabbinic in origin]. (General Principle 2), the Rambam states that all of the 613 mitzvot are explicitly mentioned in the Torah and are not dependent on the exegesis of a verse according to the oral tradition. Therefore, in Sefer HaMitzvot, he elaborates in the explanation why this prohibition can be considered a negative commandment of the Torah even though the Torah does not explicitly mention it.