It is a positive commandment of the Torah to relate the miracles and wonders wrought for our ancestors in Egypt on the night of the 15th of Nissan, as it is written (Exodus 13:3): "Remember this day when you left Egypt," similar to the obligation (Exodus 20:8): "Remember the Sabbath day."
How do we know this is on the night of the 15th? This is taught by the words (Exodus 13:8): "And you shall tell your child on that day, saying: 'It is because of this..." which indicate it is to be done at the time when matzah and marror are placed before you.
This mitzvah applies even when one does not have a child to tell. Even wise sages who know it well are obligated to tell about the Exodus from Egypt. Whoever elaborates upon the events which transpired is worthy of praise.
(Mishneh Torah, Chametz and Matzah 7:1)
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