במדבר י״ז:ט׳-ט״ו

וכפר עליהם. רָז זֶה מָסַר לוֹ מַלְאַךְ הַמָּוֶת כְּשֶׁעָלָה לָרָקִיעַ, שֶׁהַקְּטֹרֶת עוֹצֵר הַמַּגֵּפָה, כִּדְאִיתָא בְמַסֶּכֶת שַׁבָּת (דף פ"ט):

And atone for them. This was the secret, the angel of death conveyed to him when he [Moshe] ascended to heaven, that incense holds back the plague, as stated in Maseches Shabbos.

ויעמד בין המתים וגו'. אָחַז אֶת הַמַּלְאָךְ וְהֶעֱמִידוֹ עַל כָּרְחוֹ, אָמַר לוֹ הַמַּלְאָךְ, הַנַּח לִי לַעֲשׂוֹת שְׁלִיחוּתִי, אָמַר לוֹ מֹשֶׁה צִוַּנִי לְעַכֵּב עַל יָדְךָ, אָמַר לוֹ אֲנִי שְׁלוּחוֹ שֶׁל מָקוֹם וְאַתָּה שְׁלוּחוֹ שֶׁל מֹשֶׁה, אָמַר לוֹ אֵין מֹשֶׁה אוֹמֵר כְּלוּם מִלִּבּוֹ אֶלָּא מִפִּי הַגְּבוּרָה, אִם אֵין אַתָּה מַאֲמִין, הֲרֵי הַקָּבָּ"ה וּמֹשֶׁה אֶל פֶּתַח אֹהֶל מוֹעֵד, בֹּא עִמִּי וּשְׁאַל, וְזֶהוּ שֶׁנֶּאֱמַר וישב אהרן אל משה; דָּ"אַ, לָמָּה בַקְּטֹרֶת? לְפִי שֶׁהָיוּ יִשְׂרָאֵל מְלִיזִים וּמְרַנְּנִים אַחַר הַקְּטֹרֶת לוֹמַר סַם הַמָּוֶת הוּא — עַל יָדוֹ מֵתוּ נָדָב וַאֲבִיהוּא, עַל יָדוֹ נִשְׂרְפוּ חֲמִשִּׁים וּמָאתַיִם אִישׁ — אָמַר הַקָּבָּ"ה, תִּרְאוּ שֶׁעוֹצֵר מַגֵּפָה הוּא וְהַחֵטְא הוּא הַמֵּמִית (מכילתא שמות י"ז):

He stood between the dead etc. He grasped the angel and held him against his will. The angel said to him, “Permit me to perform my duty.” He said to him, “Moshe ordered me to stop you.” He said to him, “I represent God, while you represent Moshe.” He said to him, “Moshe says nothing of his own volition, only from the mouth of the Almighty. If you don’t believe me — why the Holy One, Blessed is He, and Moshe are at the entrance to the Tent of Meeting. Come with me and ask.” This is [the meaning of] what is said — Aharon returned to Moshe. Another interpretation: Why with incense? Because the Israelites were grumbling, and vilifying the incense, saying that it was a deadly poison — because of it, Nadav and Avihu died, because of it, two hundred and fifty men were consumed. The Holy One, Blessed is He, said, ‘You will see that it [incense] halts the plague — it is sinfulness, that kills.’