"And the children of Israel went and did" (Exodus 12:28). This is to give reward for the doing and reward for the going. "And they did." But had they already done it? Rather, from here we learn that they took it upon themselves to do it, and the Omnipresent reckoned it for them as though they had already done it. "As the LORD commanded Moses and Aaron, so they did." This is to make known the praise of Israel: that just as Moses and Aaron told them, so they did. Another interpretation: What does the verse teach by saying "so they did"? Rather, that Moses and Aaron also did so.
They Went and They Did and the Praise of Israel
Yalkut Shimoni on Torah 208:3
(שמות יב כח) וַיֵּלְכוּ וַיַּעֲשׂוּ בְּנֵי יִשְׂרָאֵל. לִיתֵּן שָׂכָר לַעֲשִׂיָּה וְשָׂכָר לַהֲלִיכָה. וַיַּעֲשׂוּ. וְכִי כְּבָר עָשׂוּ, אֶלָּא מִכָּאן שֶׁקִּבְּלוּ עֲלֵיהֶן לַעֲשׂוֹת וְהֶעֱלָה עֲלֵיהֶם הַמָּקוֹם כְּאִלּו עָשׂוּ. כַּאֲשֶׁר צִוָּה ה' אֶת מֹשֶׁה וְאֶת אַהֲרֹן כֵּן עָשׂוּ. לְהוֹדִיעַ שִׁבְחָן שֶׁל יִשְׂרָאֵל. שֶׁכְּשֵׁם שֶׁאָמַר לָהֶם מֹשֶׁה וְאַהֲרֹן כֵּן עָשׂוּ. דָּבָר אַחֵר, מַה תַּלְמוּד לוֹמַר כֵּן עָשׂוּ, אֶלָּא מֹשֶׁה וְאַהֲרֹן כֵּן עָשׂוּ.