Negative commandment · Lo Ta’aseh #334 of 613
Not to have intimate relations with one's son's daughter
שלא לגלות ערוות בת הבן
The verse · Vayikra 18:10
עֶרְוַת בַּת־בִּנְךָ אוֹ בַת־בִּתְּךָ לֹא תְגַלֶּה עֶרְוָתָן כִּי עֶרְוָתְךָ הֵנָּה
“The nakedness of thy son’s daughter, or of thy daughter’s daughter, their nakedness thou shalt not uncover: for theirs is thy own nakedness.”
The Rambam’s words
Not to have intimate relations with one's son's daughter, as [Leviticus 18:10] states: "[Do not commit] incest with your son's daughter."
שלא לגלות ערוות בת הבן, שנאמר "ערוות בת בנך" (ויקרא יח,י).
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