Deuteronomy 7:2-4

2and when the LORD thy God shall deliver them up before thee, and thou shalt smite them; then thou shalt utterly destroy them; thou shalt make no covenant with them, nor shew mercy unto them:

3neither shalt thou make marriages with them; thy daughter thou shalt not give unto his son, nor his daughter shalt thou take unto thy son.

4For he will turn away thy son from following me, that they may serve other gods: so will the anger of the LORD be kindled against you, and he will destroy thee quickly.