Rashi on Exodus 12:17/רש"י על שמות י״ב:י״ז

1ושמרתם 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).

2ושמרתם את היום הזה AND YE SHALL GUARD THIS DAY — against work,

3לדרתיכם חקת עולם THROUGHOUT YOUR GENERATIONS, AN ORDINANCE FOR EVER — Because the expressions, “[throughout your] generations”, and “an ordinance for ever” have not been mentioned in connection with the prohibition of work but only in reference to the celebration of the Festival (v. 14), it is therefore repeated here in order that you should not say: the prohibition, (v. 16) “no work shall be done”, was not spoken for future generations but for that generation alone to whom the words were addressed.