וְהָ֣יוּ לִ֗י אָמַר֙ יְהוָ֣ה צְבָא֔וֹת לַיּ֕וֹם אֲשֶׁ֥ר אֲנִ֖י עֹשֶׂ֣ה סְגֻלָּ֑ה וְחָמַלְתִּ֣י עֲלֵיהֶ֔ם כַּֽאֲשֶׁר֙ יַחְמֹ֣ל אִ֔ישׁ עַל־בְּנ֖וֹ הָעֹבֵ֥ד אֹתֽוֹ׃
און זײ װעלן מיר זײַן אַ טײַער אײגנס, זאָגט גאָט פֿון צבֿאוֹת, אין דעם טאָג װאָס איך באַשער; און איך װעל זיך דערבאַרימען אױף זײ, װי אַ מאַן דערבאַרימט זיך אױף זײַן זון װאָס דינט אים.
Rashi
for that day when I make a treasure that I have stored and put away, with which to pay My reward. There I will show you what the difference is between a righteous man and a wicked man.
Tze'enah Ure'enah
“They shall be Mine, says the Lord of Hosts on the day, etc.” [3:17]. It will be in that day, on the day that I will judge the evildoers, the righteous will be a people praised by Me and I will have mercy on them, like a father has compassion on his son that has been faithful to his father.