וַיֹּ֣אמֶר לָהֶ֗ם כֹּֽה־אָמַ֤ר יְהוָה֙ אֱלֹהֵ֣י יִשְׂרָאֵ֔ל שִׂ֥ימוּ אִישׁ־חַרְבּ֖וֹ עַל־יְרֵכ֑וֹ עִבְר֨וּ וָשׁ֜וּבוּ מִשַּׁ֤עַר לָשַׁ֙עַר֙ בַּֽמַּחֲנֶ֔ה וְהִרְג֧וּ אִֽישׁ־אֶת־אָחִ֛יו וְאִ֥ישׁ אֶת־רֵעֵ֖הוּ וְאִ֥ישׁ אֶת־קְרֹבֽוֹ׃
И сказал им: 'Так говорит Господь, Бог Израилев: положи каждый меч свой на свое бедро и ходи туда и обратно от ворот к воротам по всему стангу, и заколи каждого человека своим братом, и каждого его спутника, и каждого его сосед.'
Rashi
כה אמר הי וגו׳ THUS SAITH THE LORD etc. — Where did he say so? In the command, (Exodus 22:19) “He that sacrificeth unto any god shall utterly be destroyed”. Thus is it stated in the Mechilta.
Chizkuni
כה אמר, “thus has said the Lord, etc.;” where is G-d recorded as having said this? In Exodus 22,19: זובח לאלוהים יחרם, “whoever sacrifices to a god shall be proscribed.”
Ramban
THUS SAITH THE ETERNAL, THE G-D OF ISRAEL. The reason for this expression383‘Elokei’ (the G-d of) Israel — the Divine Name which denotes the attribute of justice. is that since the worshippers of the calf had intended to sacrifice to the G-d of Israel, therefore the attribute of justice extracted punishment from them, because they “mutilated the shoots” [of faith],384See Note 308 above. and besides, for the judgment is G-d’s.385Deuteronomy 1:17. It is for this reason that he [Moses], said of the Levites, for the G-d of Israel hath separated you from the congregation of Israel,386Numbers 16:9. for the Service is to the G-d of Israel, and to His Name they were separated by virtue of this meritorious deed.