Deuteronomy 7:2-4

Revised Standard Version

² and when the Lord your God gives them over to you, and you defeat them; then you must utterly destroy them; you shall make no covenant with them, and show no mercy to them. ³ You shall not make marriages with them, giving your daughters to their sons or taking their daughters for your sons. ⁴ For they would turn away your sons from following me, to serve other gods; then the anger of the Lord would be kindled against you, and he would destroy you quickly.