The 157th mitzvah is that we are commanded to tell about the Exodus from Egypt on the beginning of the night of the 15th of Nissan — with all the eloquence at our command. Praiseworthy is the person who gives lengthy discussion and description to the subject: how the Egyptians sinned against us, and what they subjected us to; how G‑d took revenge upon them; thanking G‑d (exalted be He) for all the kindness he has bestowed upon us. This is as the Sages said,1 "Everyone who discusses the Exodus from Egypt at length is praiseworthy."
The source of this commandment is G‑d's statement (exalted be He),2 "And you shall tell your child [on that day, 'It is because of this that the L‑rd did for me when I left Egypt.' "]
The Sages3 explain this verse: " 'And you shall tell your child': One may think that [the discussion of the Exodus] must be from the first of the month. The Torah therefore says, 'on that day.' But 'on that day' could mean while it is yet daytime; the Torah therefore says, 'It is because of this.' The expression 'because of this' can only be said when matzah and maror are placed before you." This means that at the beginning of the night one is obligated to discuss [the Exodus].
The Mechilta4 says: "Since the verse5 says, 'when your son will ask you,' you might think that you are required to discuss it only when he asks, and not otherwise. Therefore the Torah tells you, 'And you shall tell your child,' even if he doesn't ask. I only know [of the obligation to discuss the Exodus] when the person has a child. How do I know [this obligation applies] when he is by himself or with others? The verse says,6 'And Moshe said to the people, "Remember this day" — that G‑d commanded us to remember the Exodus just as He commanded us,7 'Remember the Shabbos day to sanctify it.' "
You are familiar with the expression [of the Sages8], "Even if all of us were wise, all of us understanding, all of us knowing the Torah, we would still be obligated to discuss the Exodus from Egypt."
The details of this mitzvah are discussed in the end of Pesachim.
