I was just learning the Mishna Berura and Aruch Hashulchan on this topic, and one thing that seemed to be strangely taken for granted by the Mishna Berura, without any source for it that I can find, was the individual's (even in a minyan with a Shliach Tzibbur) recitation of the word "Emes" at the end of Shema. The Aruch Hashulchan seems to consider an opinion which agrees that the individual shouldn't say it because it would lead to 249 words instead of 248 since the Chazzan repeats the words HE"E. However, when the M"B mentions this issue, he seems to only consider the rectification of leaving out the first Emes for the Shliach Tzibbur alone, while the rest of the Tzibbur would seemingly be left with 249. I must be misunderstanding the M"B. Either way, like the title of this question, I'd also like to know, what is the source for the halacha of adding on Emes anyway? The midrash that is brought in the Mishna Berura seems to only address the repetition of the three words HE"E.?
To address a point of terminology that may clear things up somewhat: the word "emet" is not really being "added" to shema at all. Rather, it is the first word of the blessing recited after shema i.e. emet veyatziv (or, at night, emet ve-emuna.)
So, why are we particular not to pause between the end of shema and the first word of the blessing emet veyatziv?
The source for this is the mishnah in Berachot 2:2:
רַבִּי יְהוּדָה אוֹמֵר, בֵּין וַיֹּאמֶר לֶאֱמֶת וְיַצִּיב לֹא יַפְסִיק.
Rabbi Yehuda says: Between VaYomer and emet veyatziv, one may not interrupt.
(See the gemara starting at the bottom of 14a for an explanation as to why.)
So the individual has to recite emet immediately following shema. (By the way, I don't think Aruch HaShulchan entertains a possibility of the individual not saying emet; like the Mishnah Berurah he seems to me to only be referring to the shaliach tzibbur.)
Now, how does such an individual get to the desired number of 248 words? I agree that this gets confusing, but it seems to me that both Aruch HaShulchan and Mishnah Berurah understand that the "emet" recited by the individual does not count towards the total, as it is part of the next blessing. On the other hand, the emet recited by the shaliach tzibbur (or one of them if he recites it twice) does count, presumably because he does so for the purposes of the getting to the correct number of words.
1This leads to the strange situation whereby people finish the last words of shema after already having started the next blessing– Double AA ♦Mar 14 at 15:18
That Gemara that you brought from Brachos is exactly what I was looking for! However, I just took a third look at the Aruch Hashulchan Siman 61 Siif 11, and I am very convince of my interpretation over yours. I request that you read it again and tell me if you have any reservations. One reason I say so is because his analysis of this issue immediately follows his mention of the "Yachid" and not the "Sh"Tz", which would imply that his following comments are about the yachid. Another reason is his suggestion that the SECOND Emes is not part of the minyan, which would mean the first definitely is Mar 14 at 15:37
Ooh. Actually, I see what you're saying! He says, "Some Poskim say... AND AFTERWARDS HE SHOULD SAY HE"E." Indeed, unless this is referring to the minhagim that hold everyone including the tzibbur says HE"E, then you'd be right that it implies he is referring to the SHATZ only. However, this understanding would be very perplexing, as once you establish the Emes of the Tzibbur to not be part of the "Shema Cheshbon", why in the world can't you say the same for the Shliach Tzibbur?! So once again, I really don't think that the Aruch Hashulchan is saying that. Rather like my other option above. Mar 14 at 15:46
I'm sorry. I was too quick to accept the Gemara you brought as a source. Do we not either way generally pause after finishing Shema, even when concluding with Emes? So how does Emes help? Is it the fact that you started the Bracha already that solves the issue of pausing? Furthermore, since this isn't so straightforward, why on earth did neither of the Shulchan Aruch, Mishne Berura, or Aruch Hashulchan bring down this halacha clearly?! Mar 14 at 15:52
Oops. I should have looked at the Gemara first, (I missed where you said to see the reason there). I'm sorry I keep dragging on over here lol! Anyway, I now see that it's different than I thought, and I need to really study that Gemara, but my last question in the previous comment is nonetheless relevant. Mar 14 at 15:57
אמת is the first word of the bracha after Shema. The halacha to say that bracha is given in the Shulchan Aruch in 66:10.
I don't see how this addresses my question. Mar 14 at 15:47
@AYALTAAROG you asked for the source of saying this word. The source is you have to say the bracha after Shema, and this is the first word of that bracha.– HeshyMar 14 at 17:06
I see you misunderstood me. I was asking what the source is for adding on the word Emes to "Hashem Elokeichem", vs just saying Emes V'yatziv... in its real form. Mar 14 at 20:05