Midrash for Numbers 19:21

3b) binyan av mishnei kethuvim (a general rule derived from two verses). viz.: The topic of the lamps (on the menorah) is not like that of sending the unclean outside (of the encampment), and the latter is not like the former. What is common to them is that they are introduced by "Tzav" ("Command") — [the lamps, (Vayikra 24:2); sending, etc., (Bamidbar 5:2)] — and apply both immediately and for future generations. [The lamps: immediately — (Bamidbar 8:3): "And Aaron did so. He kindled its lamps towards the face of the menorah, etc."; for future generations — (Vayikra 24:3): "… an eternal statute throughout your generations." Sending the unclean outside: immediately — (Bamidbar 5:4): "And the children of Israel did so, and they sent them outside the camp"; for future generations — (Bamidbar 19:21): "And it shall be for them an everlasting statute."] So, all commandments introduced by "Tzav" apply both immediately and for future generations.

(Bamidbar 19:21) "And it (the sprinkling of the waters) shall be for them for an everlasting statute": that it be observed throughout the generations.