מדרש על במדבר 17:17

(Exodus 14:1-2) "And the L rd said to Moses, saying: Speak to the children of Israel that they return and encamp": R. Shimon b. Yochai says: Wherever it is written "saying and tell them," it (i.e., the command that follows) is for all generations, and wherever it is not written, (as in this instance) it is for that time alone. Rebbi says: Even where it is not written, it is for all generations, except for three instances, here, (Ibid. 25:1), and (Numbers 17:17).