כַּאֲשֶׁ֤ר לֹֽא־שָׁמַ֙עְתָּ֙ בְּק֣וֹל יְהוָ֔ה וְלֹֽא־עָשִׂ֥יתָ חֲרוֹן־אַפּ֖וֹ בַּעֲמָלֵ֑ק עַל־כֵּן֙ הַדָּבָ֣ר הַזֶּ֔ה עָשָֽׂה־לְךָ֥ יְהוָ֖ה הַיּ֥וֹם הַזֶּֽה׃
Because thou didst not hearken to the voice of the LORD, and didst not execute His fierce wrath upon Amalek, therefore hath the LORD done this thing unto thee this day.
Find Mishnaic references to I Samuel 28:18: passages from the 63 tractates of the Mishnah that cite or discuss this verse, free.