Berakhot 13B

״אשר אנכי מצוך היום על לבבך״. מכאן אתה למד שכל הפרשה כולה צריכה כוונה.

“Which I command you this day, will be upon your heart.” Surely the word these, does not come to limit the mitzva of intent. On the contrary, from here you derive that the entire portion requires intent.

אמר רבה בר בר חנה אמר רבי יוחנן: הלכה כרבי עקיבא.

Rabba bar bar Ḥana said that R' Yoḥanan said: The halakha is in accordance with the opinion of R' Akiva; the entire portion requires intent.

איכא דמתני לה אהא דתניא. הקורא את שמע צריך שיכוין את לבו, רבי אחא משום רבי יהודה אומר: כיון שכוון לבו בפרק ראשון שוב אינו צריך. אמר רבה בר בר חנה אמר רבי יוחנן: הלכה כרבי אחא שאמר משום רבי יהודה.

Some teach this halakha stated by R' Yoḥanan with regard to that which was taught in a Tosefta, where there is a tannaitic dispute. The first tanna holds: One who recites Shema must focus his heart for the entire Shema. R' Aḥa says in the name of R' Yehuda: Once he focused his heart for the first paragraph alone, he no longer requires intent. With regard to this Tosefta, Rabba bar bar Ḥana said that R' Yoḥanan said: The halakha is in accordance with R' Aḥa who said in the name of R' Yehuda. While this differs from the previous version in form, it arrives at the same conclusion.

תניא אידך. ״והיו״⁠ — שלא יקרא למפרע. ״על לבבך״⁠ — רב זוטרא אומר עד כאן מצות כוונה, מכאן ואילך מצות קריאה. רבי יאשיה אומר: עד כאן מצות קריאה, מכאן ואילך מצות כוונה.

It was taught in another baraita on this subject, which cited different opinions. From: And they will be, recited in Shema, it is derived that it may not be recited out of order. From: Upon your heart, Rav Zutra says: To this point, there is the mitzva of intent; from here on, beginning with the second paragraph, there is only the mitzva of recitation. R' Yoshiya says that it means the opposite: To this point, there is the mitzva of recitation; from here on there is only the mitzva of intent.