The Talmud (Kiddushin 68b) derives this law from the Torah, specifically: Thy daughter thou shalt not give to his son, nor shalt thou take his daughter to thy son.
The Talmud (Kiddushin 68b) derives this law from the Torah. The relevant Torah passage (Deut.7:3-4) reads: Thy daughter thou shalt not give to his son, nor shalt thou take his daughter to thy son.