Rashi on Leviticus 18:10/רש"י על ויקרא י״ח:י׳

1ערות בת בנך THE NAKEDNESS OF THE DAUGHTER OF THY SON [OR THE DAUGHTER OF THY DAUGHTER THOU SHALT NOT UNCOVER] — When it states "thy daughter״ Scripture is speaking of his daughter born of a woman whom he had outraged (similarly "the daughter of thy son" means, the son born of such a woman). This must be so, for that one is forbidden to marry one's daughter or the daughter of one's daughter who was born of one's wife we may learn from the prohibition (v. 17) in the verse beginning אשה ובתה ערות "the nakedness of a woman and her daughter" of whom it is stated "thou shalt not uncover" and which thus implies the prohibition to marry one's daughter (and forbids one's marriage with one's daughter's daughter; for Scripture continues there: “neither shalt thou take her son's daughter or her daughter's daughter) whether she be begotten by him or by another man (cf. Yevamot 22b and Rashi von v. 17).

2ערות בת בנך THE NAKEDNESS OF THY SON'S DAUGHTER [… THOU SHALT NOT UNCOVER] — Since even thy son's daughter (the former being born from an אנוסה) is forbidden, the prohibition concerning thy own daughter born of thy אנוסה could be derived by a conclusion a minori ad majus (ק״ו). But because there is a rule אין מזהירין מן הדין “no prohibition can be derived by a logical conclusion" they (the Rabbis) derived it in Treatise Yevamot 3a by an analogy (גזירה שוה) based on the similar expression הנה — הנה used here and in v. 17.