מדרש אויף שמות 14:1

(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). "that they return and encamp before Pi Hachiroth": What were these "chiroth"? (Two rocks, which were) not slanting, but straight; not open (between them), but surrounded (by stones, not affording passage between them); not round, but square; not man-made, but Heaven-made; with (the figure of) open eyes; one (in the figure of) a male, and (the other in the figure of) a female. These are the words of R. Eliezer. R. Yehoshua says: Hachiroth on one side, Migdol on the other side; the sea before them and Egypt behind them.

Rabbi Shimon son of Yocahi said: all places that it says "saying" and then says "to them" it is for the generations and all places that it doesn't say "to them" it is only for that time. Rebbi said: even if it doesn't say "saying" but says "to them" it is for the generations except for 3 places.