Berakhot 39B

אלא אמר רבא: מברך ואחר כך בוצע.

Rather, Rava said: When breaking bread, one recites the blessing over the complete loaf and only afterwards, he breaks it.

נהרדעי עבדי כרבי חייא ורבנן עבדי כרבא. אמר רבינא: אמרה לי אם, אבוך עביד כרבי חייא, דאמר רבי חייא: צריך שתכלה ברכה עם הפת. ורבנן עבדי כרבא. והלכתא כרבא, דאמר מברך ואחר כך בוצע.

The Talmud relates: The rabbis of Neharde’a acted in accordance with the opinion of R' Ḥiyya and would recite the blessing as they were breaking the bread and conclude the blessing as he finished breaking off the piece of bread. And the Rabbis acted in accordance with the opinion of Rava and would recite the blessing before breaking the bread. Ravina said: My mother told me: Your father acted in accordance with the opinion of R' Ḥiyya, as R' Ḥiyya said: The blessing must conclude with the beginning of the breaking of the loaf of bread. And the Rabbis acted in accordance with the opinion of Rava. The Talmud concludes: The halakha is in accordance with the opinion of Rava, who said one recites the blessing over the complete loaf and only afterwards he breaks it.

איתמר: הביאו לפניהם פתיתין ושלמין, אמר רב הונא: מברך על הפתיתין ופוטר את השלמין. ורבי יוחנן אמר: שלמה מצוה מן המובחר. אבל פרוסה של חטין ושלמה מן השעורין, דברי הכל מברך על הפרוסה של חטין, ופוטר את השלמה של שעורין.

It was stated that there was an amoraic dispute with regard to whether to recite the blessing over a whole loaf of bread or to recite it over a piece of bread: If they brought pieces and whole loaves of bread before those partaking of a meal, Rav Huna said: One may recite the blessing over the pieces and with that blessing exempts the whole loaves as well. R' Yoḥanan said: The optimal manner in which to fulfill the mitzva is to recite the blessing over the whole loaf. However, if the piece was of wheat bread and the whole loaf was of barley bread, everyone agrees that one recites a blessing over the piece of wheat bread. Although it is a piece of bread, it is nevertheless of superior quality, and in so doing one exempts the whole loaf of barley bread.