Gemara

(Beraisa - R. Meir): In Yericho, they were Korech Shma Yisrael. Chachamim approved;

R. Yehudah: If they approved, people would do so everywhere! Rather, Chachamim disapproved, but they did not protest.

Pesachim 55b (Mishnah): In Yericho, they were Korech Shma. Chachamim did not protest.

56a - Question: What did they do?

Answer #1 (Rav Yehudah): They said the first verse without interrupting.

(Rava): They interrupted, but they said "ha'Yom" together with "Al Levavecha", which connotes that today one must put Torah to one's heart today, but tomorrow he need not.

(Beraisa) Question: How were they Korech Shma?

Answer #1 (R. Meir): They said the first verse without interrupting.

Answer #2 (R. Yehudah): They interrupted, but they did not say 'Baruch Shem Kevod Malchuso l'Olam va'Ed.'

Berachos 15b (Rav Ovadyah - Beraisa): "V'Limadtem" teaches that your recital must be Tam (pure). You must leave space Bein ha'Devekim;

(Rava): Some examples are "Al Levavecha," Esev b'Sadcha," "ha'Kanaf Pesil"...

(R. Chama b'Rebbi Chanina): Gehinom is cooled off for one who articulates the letters -- "b'Fares Shakai Melachim Bah Tashleg b'Tzalmon." We read this like 'b'Faresh... b'Tzalmaves.' (If you will separate the words (of accepting Malchus Shamayim), Hash-m will cool off Gehinom.)

Rishonim

Rif (Berachos 8b): One must leave space Bein ha'Devekim, e.g. b'Chol "Al Levavecha"...(and Oso u'Zkartem, Tikzeru va'Asisem).

Rashi (56a DH v'Lo): In Yericho they did not separate between "Echad" and "vhv". One must prolong Echad, and separate between accepting Malchus Shomayim and other things. In the first verse, everyone accepts on himself 'one, He is our G-d.' The second verse is a command.

R. Yonah (DH Omrim): R. Meir says that they did not say Shma b'Nachas (without rushing) with the Ta6 (notes, i.e. cantillation), the way we say it nowadays. The primary Mitzvah is to recite Shma with the ta6. R. Yehudah says that they said Shma b'Nachas (without rushing). The Kerichah was wrapping Shma with v'Ahavta. They would not say Baruch Shem Kevod Malchuso l'Olam Va'ed. Rashi explains that R. Meir holds that they did not separate at all between 1q and v'Ahavta. They did not say Baruch Shem Kevod, and they did not prolong the Dalet (of 1q). This is difficult. If so, R. Yehudah should have said 'they did not prolong.' Why did he say 'they did separate'? Rather, my first Perush is correct.

The Rosh (Berachos 2:12) brings Rashi's Perush.

Rambam (Hilchos Keri'as Shema 2:9): One must leave space Bein ha'Devekim, between two similar letters, one at the end of a word and the other at the beginning of the next word. E.g. in "b'Chol Levavecha", one says b'Chol, pauses, and says Levavecha.

Ra'avad: One must separate the end of "Nishba" from Hash-m, lest the Aleph (at the beginning of the way we pronounce Hash-m) be swallowed into the Ayen.

Rosh (ibid., and Hagahos Maimoniyos 10): The Yerushalmi says that one must separate between "v'Charah" and "Af", lest it sound like 'v'Charaf' (he blasphemed).

Rebuttal (Yad ha'Melech on the Rambam): The Yerushalmi does not say so. In Megilah, it lists v'Charah Af among words that are separate, e.g. Beis Kel, Poti Fera... I.e. we write them like two words. Even though they are like one word, the tradition is to write them divided into two words. Also Maseches Sofrim lists Charah Af and v'Charah Api among names that are not separate. This has no connection to articulating the words of Keri'as Shma.

Rashi (15b DH Bein): When a word begins with the same letter as the end of the previous word, if one does not leave space between them, he will read both letters like one.

Tosfos (56a DH Lo): They did not interrupt between "Shma Yisrael" and Hash-m. It sounds like one tells Hash-m to hear Yisrael, and answer them.

Poskim

Shulchan Aruch (OC 61:14): One must pause a little between l'Olam Va'ed and v'Ahavta, to separate between accepting Malchus Shomayim and other Mitzvos.

Beis Yosef (DH Kasav): Rav Amram (Gaon) says that one must separate between words of Shma. Chachamim disapproved of being Korech Shma. The Yerushalmi explains that they did not separate the words. Mahari Avuhav says that this is unlike the Bavli, which requires separating only Bein ha'Devekim. However, one can say that one must separate more Bein ha'Devekim than between other words.

Beis Yosef (DH v'Tzarich): The Tur holds that since we find that one must separate between accepting Malchus Shamayim and other things, we, who say Baruch Shem Kevod, must separate between it and v'Ahavta. Baruch Shem Kevod is part of accepting Malchus Shamayim. The Roke'ach says that one must separate between Hash-m Echad and Baruch Shem. He holds that Baruch Shem is not acceptance of Malchus Shamayim as much as the first verse.

Kaf ha'Chayim (48): When saying v'Ahavta, one should intend to full loving Hash-m. It is one of the 248 Mitzvos.

Rema: One should pause in the first verse between "Yisrael" and Hash-m, and between "Elokeinu" and Hash-m, in order that it will sound like 'hear Yisrael, that Hash-m, who is our G-d, He, Hash-m, is one.'

Kaf ha'Chayim (48): Be'er Mayim Chayim and Kitzur Shlah say to say the verse like this, in three parts. This is a full acceptance of Malchus Shamayim. Eliyahu Rabah says that the Shlah says to pause also between Shma and Yisrael, and between Hash-m and Echad.

Rema: One should pause a little between "Echad" and "Baruch", for the primary acceptance of Malchus Shamayim is in the first verse.

Shulchan Aruch (15): One must separate between "ha'Yom" and "Al Levavecha", and between "ha'Yom" "l'Ahavah", lest it seem like 'today, but not tomorrow.'

Gra: The latter is from the Tur. The Gemara merely mentioned one of them (and it is understood that the same applies to the other).

Shulchan Aruch (16): One must separate between "Nishba" and Hash-m, in order to pronounce the Ayen well, lest it seem like a Hei.

Source (Beis Yosef DH Yerushalmi): This is from the Yerushalmi.

Kaf ha'Chayim (51): If the Ayen seems like a Hei, it sounds as if He is taken captive, Chas v'Shalom.

Shulchan Aruch (19): One must separate between "v'Charah" and "Af", lest it sound like 'v'Charaf'.

Shulchan Aruch (20): One must leave space after a word that ends like the beginning of the next word, e.g. "b'Chol Levavecha", "Esev b'Sadcha", "ha'Kanaf Pesil"...

Beis Yosef (DH v'Chasav, citing the Radak): Chachamim said that one must leave space Bein ha'Devekim. They did not say to interrupt, and not put a Makaf where there is a Makaf, e.g. between the 'Lamed'im of "b'Chol Levavecha." (Note: a Makaf (dash) between words shows that one reads both of them with one Ta'am.) Rather, even though he reads them with a Makaf, he leaves space and separates between them. Due to the Makaf, b'Chol has a Kamatz (under the Chaf). If not for the Makaf, it would be a Cholem. The Avudraham brought this.

Sha'arei Teshuvah (12): Shalmei Tzibur brings from Shtei Yados that in order to say together the words with a Makaf in between, one should stop a little in the word before them. If one stops a little in v'Nasati, it is easy to say Metar Artzechem with a Makaf. Maharam Nigrin saw written that one should pause between Af and Hash-m. He explained that this is lest it sound like Hash-m is Af. I say that we do not heed people who fabricate breaks on their own. One should not pause between them, for there is a Makaf between them! Perhaps Maharam Nigrin means that one should articulate these words well, lest it sound like Afdonai. Perhaps we are concerned lest it sound as if Af (i.e. even, and not 'anger') Hash-m will be enraged. Since one must pause between v'Charah" and "Af", lest it sound like 'v'Charaf'', one should not say Hash-m too soon after Af. However, there should not be a noticeable break in between.

Kaf ha'Chayim (56): One has a hot nature, to read quickly. If he cools it to read precisely, Midah k'Neged Midah (correspondingly) Hash-m cools off Gehinom, unlike its nature. If one is not careful, letters will be lacking and blemished.

Mishnah Brurah (62:2). Because he activated his own heat to be meticulous, Hash-m cools off another heat for him.

Kaf ha'Chayim (57): Likewise, one must be careful about a double letter within a word, e.g. Levavecha, lest one pronounce the letter only once.

Shulchan Aruch (21): One must separate every word beginning with Aleph after a word ending with Mem, e.g. "v'Limadtem Osam (Es)," lest it sound like Mosam (Mes).

Source (Beis Yosef DH u'Mah): R. Yonah says so in Sefer ha'Yir'ah.

Mishnah Berurah (34): Likewise, one should pause before any word beginning with Aleph that might not be Nirgash (recognized to be a new syllable), e.g. Metar Artzechem, Devarai Eleh, Mitzvos Hash-m, and everything similar.

Note: If one pronounces Ayen like Aleph, this applies also to words that begin with Ayen.

Mishnah Berurah (34): Likewise, one should stress a Yud at the beginning of a word, lest it seem like part of the previous word, e.g. Pen Yifteh should not sound like Penyifteh or Pen Ifteh. The same applies to Yirbu and all such words.

Shulchan Aruch (22): Even in Pesukei d'Zimrah and Tefilah, one must be meticulous about this.

Source (Beis Yosef DH u'Mah): R. Yonah says so in Sefer ha'Yir'ah.

Rema: Likewise, one who reads Torah, Nevi'im and Kesuvim should be careful.

Beis Yosef (DH veha'Rav, citing the Radak): Chachamim warned about this regarding Keri'as Shma, and one should likewise be careful about anything he reads in Torah, Nevi'im and Kesuvim. They discussed Keri'as Shma because these Parshiyos contain Hash-m's unity and accepting Ol Malchus Shamayim, and also because all of Yisrael read them twice every day, Chachamim and Amei ha'Aretz. They needed to warn the Amei ha'Aretz, who are not expert about reading.