וְאֶת־כָּל־שְׁלָלָ֗הּ תִּקְבֹּץ֮ אֶל־תּ֣וֹךְ רְחֹבָהּ֒ וְשָׂרַפְתָּ֨ בָאֵ֜שׁ אֶת־הָעִ֤יר וְאֶת־כָּל־שְׁלָלָהּ֙ כָּלִ֔יל לַיהוָ֖ה אֱלֹהֶ֑יךָ וְהָיְתָה֙ תֵּ֣ל עוֹלָ֔ם לֹ֥א תִבָּנֶ֖ה עֽוֹד׃
And thou shalt gather all the spoil of it into the midst of the broad place thereof, and shall burn with fire the city, and all the spoil thereof every whit, unto the LORD thy God; and it shall be a heap for ever; it shall not be built again.
Mesilat Yesharim
They further said: "'And he shall give you mercy, and have mercy upon you' (Devarim 13:17) - anyone who is merciful to others, will be shown mercy by Heaven" (Shabbat 151b).