Negative commandment · Lo Ta’aseh #336 of 613
To not reveal the nakedness of the daughter
שלא לגלות ערוות הבת; ולמה לא נתפרשה בתורה, מפני שאסר בת הבת שתק מן הבת, ומפי השמועה למדו שאיסור הבת מגופי תורה, כשאר עריות
The verse · Vayikra 18:17
עֶרְוַת אִשָּׁה וּבִתָּהּ לֹא תְגַלֵּה אֶת־בַּת־בְּנָהּ וְאֶת־בַּת־בִּתָּהּ לֹא תִקַּח לְגַלּוֹת עֶרְוָתָהּ שַׁאֲרָה הֵנָּה זִמָּה הִוא
“Thou shalt not uncover the nakedness of a woman and her daughter, neither shalt thou take her son’s daughter, or her daughter’s daughter, to uncover her nakedness; for they are her near kinswomen: it is foulness.”
The Rambam’s words
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.
שלא לגלות ערוות הבת; ולמה לא נתפרשה בתורה, מפני שאסר בת הבת שתק מן הבת, ומפי השמועה למדו שאיסור הבת מגופי תורה, כשאר עריות.
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה