כֹּ֚ה אָמַ֣ר יְהוָ֔ה עַל־שְׁלֹשָׁה֙ פִּשְׁעֵ֣י בְנֵֽי־עַמּ֔וֹן וְעַל־אַרְבָּעָ֖ה לֹ֣א אֲשִׁיבֶ֑נּוּ עַל־בִּקְעָם֙ הָר֣וֹת הַגִּלְעָ֔ד לְמַ֖עַן הַרְחִ֥יב אֶת־גְּבוּלָֽם׃
Thus saith the LORD: For three transgressions of the children of Ammon, yea, for four, I will not reverse it: because they have ripped up the women with child of Gilead, that they might enlarge their border.
Read midrashic interpretation of Amos 1:13: and classical rabbinic exegesis.