Rashi on Deuteronomy 21:10/רש"י על דברים כ״א:י׳

1כי תצא למלחמה WHEN THOU GOEST FORTH TO WAR — Scripture is speaking here of a war that is not obligatory upon the Israelites (Sifrei Devarim 211:1), for in regard to a war that was waged against the inhabitants of Erez Israel, Scripture could not possibly say, “and thou hast captured captives”, since it has already been stated regarding them, (Deuteronomy 20:16) “[But of the cities of those people ...] you shall not allow any soul to live”.

2ושבית שביו AND THOU HAST CAPTURED CAPTIVES — These apparently redundant words are intended to include Canaanite people living in it (in a city outside Canaan), that it is allowed to capture them although they belong to the seven nations (Sifrei Devarim 211:4; Sotah 35b).