One Mitzva Fulfilled for All Klal Yisroel

Rav Yehoshua of Belz pointed out that the pasuk in Parshas Bo begins in the singular and concludes in the plural: "In order that you relate to your son and grandson," and then, "In order that you all know that I am Hashem." He explained that the mitzva of Sippur Yetzias Mitzrayim, retelling the story of the Exodus, which we fulfill through reading the Haggoda at the Pesach Seder, was already incumbent upon Bnei Yisroel in the very first year that it was commanded. However, only Moshe could fulfill it, because only his wife and sons were not exiled in Egypt. Because Moshe's neshoma included all the souls of Klal Yisroel, he was able to fulfill the mitzva on behalf of them all. This is why the pasuk begins in the singular, referring to Moshe, and concludes in the plural.