Parashas Shemos
וַיִּֽחַר־אַ֨ף יְהֹוָ֜ה בְּמֹשֶׁ֗ה וַיֹּ֙אמֶר֙ הֲלֹ֨א אַהֲרֹ֤ן אָחִ֙יךָ֙ הַלֵּוִ֔י יָדַ֕עְתִּי כִּֽי־דַבֵּ֥ר יְדַבֵּ֖ר ה֑וּא וְגַ֤ם הִנֵּה־הוּא֙ יֹצֵ֣א לִקְרָאתֶ֔ךָ וְרָאֲךָ֖ וְשָׂמַ֥ח בְּלִבּֽוֹ׃
GOD became angry with Moses and said, “There is your brother Aaron the Levite. He, I know, speaks readily. Even now he is setting out to meet you, and he will be happy to see you.
Torah Temimah points to R’ Malai — Shabbos 139a (verified).
וראך ושמח בלבו — א״ר מלאי: בשכר וראך ושמח בלבו זכה לחשן המשפט על לבו.
English: “And he will see you and rejoice in his heart” — R’ Malai said: In the merit of “he will see you and rejoice in his heart” he merited the breastplate of judgment upon his heart.
Custom
Chazal teach that Aharon’s sincere joy at Moshe’s role earned him the breastplate worn over the heart.