Deuteronomy 20:13-16

13The Lord your God will deliver it over to you and you must kill every single male by the sword.

14However, the women, little children, cattle, and anything else in the city – all its plunder – you may take for yourselves as spoil. You may take from your enemies the plunder that the Lord your God has given you.

15This is how you are to deal with all those cities located far from you, those that do not belong to these nearby nations.

16As for the cities of these peoples that the Lord your God is going to give you as an inheritance, you must not allow a single living thing to survive.