Commentaire sur L’Exode 12:17

ושמרתם את המצות AND YE SHALL WATCH THE UNLEAVENED BREAD that it shall not reach the stage of becoming leavened; hence the Rabbis said, if it (the dough) is rising (a sign that the leavening process is setting in) she (the woman kneading the dough) polishes it with cold water (i. e. she slaps the dough with hands dipped in cold water). Rabbi Josiah said: Do not read “את המַּצּוֹת”, the unleavened bread, but את הַמִּצְוֹת “[ye shall watch] the commandements” — just as we may not cause the unleavened bread to become leavened by letting the dough remain in its raw state too long so we may not let the commandment become “leavened” by waiting too long before we perform it; but if it (a commandment) comes to your hand, perform it immediately (Mekhilta d'Rabbi Yishmael 12:17:1).

ושמרתם את המצות, which are called such to reflect the speed with which they have been baked without giving the dough a chance to ferment and rise.

ושמרתם את המצות, to eat them on this day as a memorial.