Rashi on Exodus 19:6/רש"י על שמות י״ט:ו׳

1ואתם 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.

2אלה הדברים THESE ARE THE WORDS — these neither fewer nor more (Mekhilta).