שמות פרק יב , כג וְעָּבַר ה' לִנְגֹף אֶׂת-מִצְרַיִם וְרָּאָּה אֶׂת-הַדָּּם עַל-הַמַשְקוֹף וְעַל שְתֵ י הַמְזוּזֹת וּפָּסַח ה' עַל-הַפֶׂתַח וְלֹא יִתֵן הַמַשְחִית לָּבֹא אֶׂל-בָּתֵיכֶׂם לִנְגֹף:
According to the Medrash, the two doorposts allude to Moshe and Aharon, and the lintel at the top refers to the Jews. These two, Moshe & Aharon are the two posts which support the lintel, the Jewish nation. Thus, the Jews’ merit resulted from their reliance on Moshe and Aharon to protect them.
When Moshe commanded this mitzvah to the Jews, he did so alone without Aharon. Thus, being humble and not wanting to accept the honor by himself, Moshe first said to put the blood on the lintel and then on the doorposts. But the Jews when they did the actual Mitzvah, honored Moshe and Aharon, and first placed the blood on the doorposts and then on the lintel. The Jews were praised for following the command as G-d said it to Moshe and Aharon (first the doorposts and then the lintel).
