This is from Deuteronomy:
Deuteronomy, chapter 7
1: "When the LORD your God brings you into the land which you are entering to take possession of it, and clears away many nations before you, the Hittites, the Gir'gashites, the Amorites, the Canaanites, the Per'izzites, the Hivites, and the Jeb'usites, seven nations greater and mightier than yourselves,
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.
5: But thus shall you deal with them: you shall break down their altars, and dash in pieces their pillars, and hew down their Ashe'rim, and burn their graven images with fire.
6: "For you are a people holy to the LORD your God; the LORD your God has chosen you to be a people for his own possession, out of all the peoples that are on the face of the earth.
16: And you shall destroy all the peoples that the LORD your God will give over to you, your eye shall not pity them; neither shall you serve their gods, for that would be a snare to you.
Deuteronomy, chapter 20
16: But in the cities of these peoples that the LORD your God gives you for an inheritance, you shall save alive nothing that breathes,
17: but you shall utterly destroy them, the Hittites and the Amorites, the Canaanites and the Per'izzites, the Hivites and the Jeb'usites, as the LORD your God has commanded;
18: that they may not teach you to do according to all their abominable practices which they have done in the service of their gods, and so to sin against the LORD your God.
