Sefer HaChinuch ספר החינוך

To not eat from anything that has chamets in it: To not eat from things that have chamets in them, even though the main part is not a chamets product — for example, Babylonian ‏kutach (a type of bread pudding) and similar to it. As it is stated (Exodus 12:20), “All that is leavened shall you not eat” — and they, may their memory be blessed, explained (Pesachim 43a) that the subject of the verse instructs about this; as so did they receive the understanding about it. And the opinion of Rambam (Sefer HaMitzvot, Negative Commandments 198), may his memory be blessed, is that if there is in these foods a kazayit (the size of a large olive) of chamets [that is eaten] in the time it takes to eat a peras (a half-loaf of bread), it is forbidden by the Torah as a negative commandment — meaning to say for lashes, but not for excision, since it is mixed with a majority [of non-chamets products]. But if there is not in them a kazayit of chamets [that is eaten] in the time it takes to eat a peras, there are no [formal] lashes, but rather [just] lashes of rebellion — as it is only prohibited rabbinically. And so is it written in his great essay (Mishneh Torah, Laws of Leavened and Unleavened Bread 1:6). And if so — according to his opinion — this prohibition of “All that is leavened” comes when a kazayit of chamets [that is eaten] in the time it takes to eat a peras, is mixed in. [This is such] that there should be a negative commandment — meaning to say [for] lashes, not excision. And Ramban, may his memory be blessed, wrote the opposite of this (on Sefer HaMitzvot, Negative Commandments 198) and said that this negative commandment is not considered [a separate commandment] among the negative commandments, but rather is one of the many negative commandments [included in the prohibition] of leaven and chamets. And this prohibition (of “All that is leavened”) is coming to teach about that which the Sages, may their memory be blessed, said in the Gemara (Pesachim 28b), “I only have if it became leavened on its own. From where [do I know] if it became leavened due to another substance? [Hence] we learn to teach, ‘All that is leavened.’” But concerning chamets that is mixed, anytime there is a kazayit of chamets [that is eaten] in the time it takes to eat a peras, there is no need for a separate negative commandment about it — as behold, it is as if it is intact, and it [comes with] excision. And if there is not a kazayit of chamets [that is eaten] in the time it takes to eat a peras, one who eats it is exempt [from the prohibition], but [still doing something] forbidden. As the law is like the words of the Sages (Pesachim 43a) that said [that] on complete leavened grain products, one is punished with excision. And anything that is a kazayit of chamets [that is eaten] in the time it takes to eat a peras is called a complete leavened grain product. And about its mixture — meaning less than a kazayit of chamets [that is eaten] in the time it takes to eat a peras — [there] is no [prohibition]. And [the law] is not like Rabbi Eliezer who disagrees with them in the Gemara and said that its mixture is a negative commandment.

From the roots of this commandment is that which we have written about leaven. However the Torah distanced it from us so much in order to strengthen the matter in our hearts.

The laws of the commandment — for example, what are the things that are included in this prohibition; what are their names; and the rest of its details — are in Pesachim. (See Tur, Orach Chaim 446.)

And [it] is practiced by males and females in every place and at all times. And one who transgresses it is lashed — and on condition that there be in it a kazayit of chamets [that is eaten] in the time it takes to eat a peras, as we have said — but there is no excision for it. [This is] according to the opinion of Rambam. But according to the opinion of Ramban, may his memory be blessed, there is excision. But if there is not in it a kazayit of chamets [that is eaten] in the time it takes to eat a peras, there is no liability for lashes. Rather it is like the law of a half size [of something prohibited], which is forbidden by the Torah, but we do not give lashes for it. And about this they, may their memories be blessed, both [agreed].