Negative commandment · Lo Ta’aseh #199 of 613
Not to eat chametz after noontime on the fourteenth [of Nisan]
שלא לאכול חמץ אחר חצות יום ארבעה עשר
The verse · Devarim 16:3
לֹא־תֹאכַל עָלָיו חָמֵץ שִׁבְעַת יָמִים תֹּאכַל־עָלָיו מַצּוֹת לֶחֶם עֹנִי כִּי בְחִפָּזוֹן יָצָאתָ מֵאֶרֶץ מִצְרַיִם לְמַעַן תִּזְכֹּר אֶת־יוֹם צֵאתְךָ מֵאֶרֶץ מִצְרַיִם כֹּל יְמֵי חַיֶּיךָ
“Thou shalt eat no leavened bread with it; seven days shalt thou eat unleavened bread with it, the bread of affliction; for thou camest forth out of the land of Miżrayim in haste: that thou mayst remember the day when thou camest out of the land of Miżrayim all the days of thy life.”
The Rambam’s words
Not to eat chametz after noontime on the fourteenth [of Nisan], as [Deuteronomy 16:3] states: "Do not eat chametz with it."
שלא לאכול חמץ אחר חצות יום ארבעה עשר, שנאמר "לא תאכל עליו חמץ" (דברים טז,ג).
Why · Sefer HaChinuch #485
To not eat chametz after midday
“It is from the roots of the commandment [that it is] because the matter of the prohibition of chametz on Pesach is an extremely weighty prohibition, from the angle that it is a great foundation in our religion; since the exodus from Egypt is a sign and wonder that necessitates [there having been] a creation of the world, which is a great pillar upon which the chambers of the Torah rest — as we have written several times.”
Who & when
“And it is practiced by males and females in all places and at all times, even at this time when we do not have the Pesach sacrifice.”
Where in the Mishneh Torah
Hilchot Leavened and Unleavened Bread הלכות חמץ ומצה