12:28י״ב:כ״ח
1 א

וילכו ויעשו בני ישראל. וְכִי כְּבָר עָשׂוּ, וַהֲלֹא מֵרֹאשׁ חֹדֶשׁ נֶאֱמַר לָהֶם? אֶלָּא מִכֵּיוָן שֶׁקִּבְּלוּ עֲלֵיהֶם מַעֲלֶה עֲלֵיהֶם הַכָּתוּב כְּאִלּוּ עָשׂוּ (מכילתא):

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).

2 ב

וילכו ויעשו. אַף הַהֲלִיכָה מָנָה הַכָּתוּב, לִתֵּן שָׂכָר לַהֲלִיכָה וְשָׂכָר לַעֲשִׂיָּה (שם):

וילכו ויעשו THEY WENT AND DID — Scripture enumerates their going also, to give them a reward for going and a reward for doing (Mekhilta d'Rabbi Yishmael 12:25).

3 ג

כאשר צוה ה' את משה ואהרן. לְהַגִּיד שִׁבְחָן שֶׁל יִשְׂרָאֵל שֶׁלֹּא הִפִּילוּ דָּבָר מִכָּל מִצְווֹת מֹשֶׁה וְאַהֲרֹן, וּמַהוּ כן עשו? אַף מֹשֶׁה וְאַהֲרֹן כֵּן עָשׂוּ (שם):

כאשר צוה ה' את משה ואהרן [THEY DID] AS THE LORD HAD COMMANDED MOSES AND AARON — This is stated for the purpose of declaring Israel’s praise — that they did not omit a single matter of all the commandments of Moses and Aaron. And what is the meaning of כן עשו SO DID THEY? Moses and Aaron also did thus!(Mekhilta d'Rabbi Yishmael 12:25).