Siman 194

Three who ate as one, and forgot to perform zimun, and each one recited the blessing for himself—the zimun is nullified for them. And they are no longer able to recite the blessing of zimun, for there is no zimun retroactively. That is to say: the zimun is prior to the rest of the blessings and not afterward, and it is "a crooked thing that cannot be made straight." And so too if two of them recited the blessing. But if only one of them recited the blessing—the two may perform zimun with him; for since he too became obligated in zimun, and the majority of the group has not yet recited the blessing, it is considered that the zimun is in its proper order and not retroactive, for we follow the majority. And he has not fulfilled the mitzvah of zimun, for there is no zimun retroactively. And should you say: how can he say "Blessed is He of whose food we have eaten," behold he already recited the blessing over his eating? For it is not so, for since he did not recite the blessing according to the halacha—it is no worse than one who ate a leaf of a vegetable. And therefore even according to the one who holds that we do not join one who ate a leaf of a vegetable, this is superior to it since he did not recite the blessing according to its halacha. (So it appears to me regarding the reason for the matter. And that which the Tur wrote, that it is no worse than a leaf of a vegetable, is not a reason for the zimun as the Magen Avraham wrote in subsection 1, but rather regarding what he says "of whose food we have eaten." And so it is explicitly in the Tur. And according to this, the reason for the law emerges properly. And the HaGaon Rav Zalman labored over this, and according to what I have written the reason is understood. Examine it and you will find it simple.) (And it has already been explained in the previous siman that when one performed zimun with others—he nullified the mitzvah of zimun for the two. And it is forbidden to do so, see there.) (The Rif omitted this law, and his reason is not known.)

Three who ate as one, and one of them went out to the market before birkat hamazon – they call to him and inform him that they wish to perform the zimmun so that he may come and intend and join with them, and answer the blessing of the zimmun with them. And they fulfill their obligation, and even if he did not sit with them at the table but rather stands before the entrance – they fulfill the obligation, and he also fulfills his obligation. And this is regarding three. But regarding ten, since they must mention the Name – it is not proper conduct that he should not sit at the table, and he is compelled to sit with them until the end of the blessing of the zimmun. And this will be explained in siman 200.

It is a known matter that regarding birkat hamazon, either everyone recites the blessing together with the one leading the blessing according to our custom, or the one leading the blessing recites it and everyone listens and answers "amen," as I wrote at the beginning of the previous siman. And when two of them are amei ha'aretz, it is necessary that only the one leading the blessing recites all the blessings, and they listen and answer "amen." But if all of them are amei ha'aretz—they have no one to recite the blessing. And our sages, of blessed memory, said (41a according to the explanation of the Rif) that three who ate as one, and there is not one among them who knows how to recite all the blessings; rather one of them knows the first blessing, and one the second blessing, and one the third blessing—they are obligated in zimun, and each one shall recite the blessing that he knows, and the two answer "amen." And so too regarding the second and third blessing. And even though there is not one among them who knows the blessing of "ha-tov ve-ha-meitiv," it does not prevent it, for this blessing is d'rabbanan. But they shall not recite it in halves, for if one knows half the blessing, and the second the second half—they shall not perform zimun, for there is no blessing in halves, and it is not divided into two. And it is obvious that they must also know the blessing of the zimun. And so too regarding two who ate, for we hold at the beginning of the previous siman that the one who knows recites the blessing and the ignoramus fulfills his obligation, if one knows one blessing or two, and the second knows the third—this one shall recite the first and the second, and the second the third, and so too anything similar to this. And all the poskim ruled this law.

And there is a question regarding this: for from this it is implied that if there is no one among them all who knows one of the three blessings – the zimun is nullified. And there is no reason for this at all, and why should they not perform the zimun over the blessing that they do know? And the answer is thus: for they are certainly obligated to recite that blessing which they know, only the mitzvah of zimun exists only when there are three blessings. For since we derive the blessing of zimun and the three blessings from the verse "And you shall eat and be satisfied and you shall bless" (48b), if so, when there are three blessings there is zimun, and when there are not three blessings there is no zimun. And from this it is proven that all three blessings are from the Torah. For if not so, why would they be indispensable? And behold, "HaTov VeHaMeitiv" which is rabbinic does not hinder. (And so wrote the Magen Avraham, subsection 3. And see there what he wrote in the name of the Ramban, that the blessings do not hinder one another, and he challenged him from here. And according to what I have written this is not a challenge, for here it was stated only regarding the matter of the blessing of zimun. And be precise and you will find it simple.)