וַיֹּ֤אמֶר יְהוָה֙ אלו [אֵלָ֔יו] עֲבֹר֙ בְּת֣וֹךְ הָעִ֔יר בְּת֖וֹךְ יְרֽוּשָׁלִָ֑ם וְהִתְוִ֨יתָ תָּ֜ו עַל־מִצְח֣וֹת הָאֲנָשִׁ֗ים הַנֶּֽאֱנָחִים֙ וְהַנֶּ֣אֱנָקִ֔ים עַ֚ל כָּל־הַתּ֣וֹעֵב֔וֹת הַֽנַּעֲשׂ֖וֹת בְּתוֹכָֽהּ׃
And the LORD said unto him: ‘Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that are done in the midst thereof.’
Rashi on Ezekiel
on the foreheads of the [righteous] men in order to be for a sign that the destroyers should not strike them.