Komentarz do Wyjścia 12:28

וילכו ויעשו בני ישראל AND THE CHILDREN OF ISRAEL WENT AND DID — But did they really do this at once — was not all this spoken to them at the beginning of the month (cf. Rashi on v. 3) and they carried out the command only on the tenth and the fourteenth? But the explanation is: so soon as they had taken these duties upon themselves Scripture accounts it unto them as though they had already performed them (Mekhilta d'Rabbi Yishmael 12:25).

AND THE CHILDREN OF ISRAEL WENT AND DID SO; AS THE ETERNAL HATH COMMANDED MOSES AND AARON, SO DID THEY. That is, they departed from before Moses and went to the sheep and slaughtered the Passover-offering at eventide [of the fourteenth day of Nisan]. Now such is Scriptural custom to repeat and say, so did they, in order to explain that they did not omit anything from whatever they were commanded, as I have explained in the case of Noah.²⁷⁰Genesis 6:22. See Vol. I, p. 114. A similar case is the verse, And Moses saw all the work, and behold, they had done it; as the Eternal had commanded, even so had they done it.²⁷¹Further, 39:42.
Now our Rabbis have a Midrash on this verse, since it was not necessary for Scripture to mention that the children of Israel “went” [and did so]. Therefore they explained:²⁷²Mechilta on the verse before us. “And the children of Israel went. This indicates that reward is given for going [to perform a religious duty] as well as for actually performing it. And they did so. And had they already done so?²⁷³“Was not all this spoken to them at the beginning ot the month?” (Rashi). They were commanded to take the lamb on the tenth day and slaughter it on the fourteenth. Thus they could not actually have done it all at once. No, but once they undertook to perform these duties, Scripture accounts it to them as if they had done them. As the Eternal hath commanded Moses and Aaron, so did they. This is to make known the praiseworthiness of Israel, i.e., that they did exactly as Moses and Aaron told them. Another interpretation: What is the meaning of the words, so did they? It is to teach us that Moses and Aaron also did so.”²⁷⁴Since one of the main purposes of the slaughtering of the lamb was a rejection of the belief in idolatry — see Ramban above, Verse 3 — one might have thought that Moses and Aaron, whose belief in the One True G-d was perfect, were not in need of taking part in this commandment. Scripture therefore informs us, according to this Mechilta, that they did as all Israel did, inasmuch as they so cherished G-d’s commandment. (From the commentary Zeh Yenachmeinu on the Mechilta, mentioned in my Hebrew work, p. 335.) The Rabbis thus expounded first that the repetition [of the phrase, so did they], was in praise of Israel, i.e., that they did not forget [all they were commanded], and that they did not omit anything of whatever was told to them. This is the customary way of the [Sacred] Language in many places.

וילכו ויעשו בני ישראל כאשר צוה השם, The children of Israel went and did as G'd commanded, etc." The Torah had to repeat כן עשו, "they did so," to underline that they obeyed what Moses had commanded them in the name of G'd though they did not understand the meaning of all these regulations. Had the Torah not written the words כן עשו, we could have thought that they did so because they understood it all.