וַיָּבֹ֨א מֹשֶׁ֤ה וְאַהֲרֹן֙ אֶל־פַּרְעֹ֔ה וַיַּ֣עַשׂוּ כֵ֔ן כַּאֲשֶׁ֖ר צִוָּ֣ה יְהוָ֑ה וַיַּשְׁלֵ֨ךְ אַהֲרֹ֜ן אֶת־מַטֵּ֗הוּ לִפְנֵ֥י פַרְעֹ֛ה וְלִפְנֵ֥י עֲבָדָ֖יו וַיְהִ֥י לְתַנִּֽין׃
Or HaChaim on Exodus
ויעשו כן כאשר צוה, They did as G'd had commanded, etc. The reason the Torah writes the words כן כאשר, i.e. repeating basically the same word is, because it tells us that Moses and Aaron a) did what G'd had told them to, b) did not do so until Pharaoh had requested the miracle from them.