Positive commandment · Mitzvat Aseh #158 of 613
To eat matzah on this night
לאכול מצה בליל זה
The verse · Shemot 12:18
בָּרִאשֹׁן בְּאַרְבָּעָה עָשָׂר יוֹם לַחֹדֶשׁ בָּעֶרֶב תֹּאכְלוּ מַצֹּת עַד יוֹם הָאֶחָד וְעֶשְׂרִים לַחֹדֶשׁ בָּעָרֶב
“In the first month, on the fourteenth day of the month at evening, shall you eat unleavened bread, until the twenty first day of the month at evening.”
The Rambam’s words
To eat matzah on this night, as [Exodus 12:18] states: "In the evening, you shall eat matzot."
לאכול מצה בליל זה, שנאמר: "בערב, תאכלו, מצות" (שמות יב,יח).
Why · Sefer HaChinuch #10
Eating matzah
“From the roots of this commandment are so that the Jews will remember forever the great miracles that G-d performed for them during the exodus from Egypt.”
Who & when
“And it is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Leavened and Unleavened Bread הלכות חמץ ומצה