וַיִּפְקֹ֥ד אֹתָ֛ם מֹשֶׁ֖ה עַל־פִּ֣י יְהוָ֑ה כַּאֲשֶׁ֖ר צֻוָּֽה׃
모세가 여호와의 말씀을 좇아 그 명하신 대로 계수하니라
Rabbeinu Bahya
A kabbalistic approach: The words זאת אשר ללוים are to be understood in the same vein as Psalms 118,23 מאת ה' היתה זאת, “This (covenant) emanated from Hashem.” We know this from Genesis 17,10: זאת בריתי, “This is My covenant.” In connection with the tribe of Levi the Torah writes ובריתך ינצורו, “they have observed Your covenant meticulously” (Deut. 33, 9). When the Levites were counted the Torah wrote the words על פי ה' (Numbers 3,16), an expression we do not find in connection with the census of the other tribes. The idea is that the מדה, the particular attribute applicable to the Levites is that they are the transmitters of the פי ה', the word of G’d, the Torah.