Leviticus 18:10

18:10 You must not expose the nakedness of your son’s daughter or your daughter’s daughter by having sexual intercourse with them, because they are your own nakedness. ¹ 

[18:10]  ¹ sn That is, to have sexual intercourse with one’s granddaughter would be like openly exposing one’s own shameful nakedness (see the note on v. 7 above).