Xuất Ai Cập Ký 19:6

ואתם תהיו לי ממלכת כהנים AND YE SHALL BE UNTO ME A KINGDOM OF כהנים — i. e. princes, just as you say, (II Samuel 8:18) “And the sons of David were princes (כהנים)”, which cannot denote priests since his sons were of the tribe of Judah and not of Levi, the priestly tribe. אלה הדברים THESE ARE THE WORDS — these neither fewer nor more (Mekhilta).

and you shall be to me a kingdom of priests, and a holy nation. These are the words which thou shalt speak to the children of Yisra᾽el.

וְאַתֶּ֧ם תִּהְיוּ־לִ֛י מַמְלֶ֥כֶת כֹּהֲנִ֖ים וְג֣וֹי קָד֑וֹשׁ אֵ֚לֶּה הַדְּבָרִ֔ים אֲשֶׁ֥ר תְּדַבֵּ֖ר אֶל־בְּנֵ֥י יִשְׂרָאֵֽל׃