Rashi on Deuteronomy 7:4/רש"י על דברים ז׳:ד׳

1כי יסיר את בנך מאחרי FOR HE WILL TURN AWAY THY SON FROM FOLLOWING ME — i.e., the son of the heathen when he marries thy daughter, will turn away thy son (grandson) whom thy daughter will bear unto him from following Me. This teaches us, that thy daughter's son who is born of a heathen is termed thy son, but thy son’s son, who is born of a heathen woman, is not termed thy son, but her son, for, you see, in regard to the statement: “his daughter thou shalt not take [to thy son]" it does not add “For she will turn away thy son (grandson) from following Me" (Yevamot 23a).