Also in Midrash Rabbah (Exodus Parsha 22:4) it is explained according to Rashi's interpretation: "Our Rabbis taught, one who recites the Shema must mention the splitting of the Red Sea, the Plague of the Firstborn, and the standing at Mount Sinai. If he did not mention them, he is not required to repeat the Shema. However, if he did not mention the exodus from Egypt, he must repeat it, as it is stated (Deuteronomy 16:3), 'That you may remember the day of your departure from the land of Egypt.'" Indeed, Rabbi Akiva Eiger's version (in his glosses on Magen Avraham Siman 67:1) in the Midrash is that if one did not mention the Plague of the Firstborn, he must repeat it. This is also the version in the Shulchan Aruch HaRav (Orach Chayim Siman 66:12). For all these opinions, an explanation is necessary as to why one must mention the Plague of the Firstborn more than the other plagues.