Pesachim 105A

קשיא לעולא! אמר לך עולא: הא נמי כברכת המצות דמיא, ברכת המצות מאי טעמא? משום דהודאה היא ⁠— הא נמי הודאה היא.

This is difficult for the opinion of Ulla, who began but did not conclude the blessing of havdala with: Blessed. The Talmud answers: Ulla could have said to you: This blessing is also considered like a blessing over mitzvot, and therefore it does not require a separate conclusion. The Talmud clarifies this response: What is the reason that blessings over mitzvot do not require a distinctive conclusion? It is because a blessing over a mitzva is a statement of praise, and as it does not include anything unrelated to the praise, e.g., a request or supplication, it is unnecessary to add a separate concluding blessing. This havdala blessing also is comprised only of praise.

רב חנניא בר שלמיא ותלמידי דרב הוו יתבי בסעודתא, וקאי עלייהו רב המנונא סבא. אמרו ליה: זיל חזי אי מקדיש יומא נפסיק וניקבעיה לשבתא. אמר להו: לא צריכיתו ⁠— שבתא קבעה נפשה.

The Talmud relates that Rav Ḥananya bar Shelemya and other students of Rav were sitting at a meal on Friday shortly before nightfall, and Rav Hamnuna the Elder was standing over them to serve them. They said to him: Go and see if the day of Shabbat has become sanctified through nightfall. If so, we will interrupt our meal by removing the tables and establish its continuation as the meal for Shabbat. Rav Hamnuna the Elder said to them: You do not need to do this, as Shabbat establishes itself. Whatever you eat after nightfall is automatically considered a Shabbat meal, even without any specific action that designates it as such.

דאמר רב: כשם שהשבת קובעת למעשר, כך שבת קובעת לקידוש.

Rav Hamnuna the Elder explained his ruling. As Rav said: Just as Shabbat establishes food consumption as a regular, set meal with regard to tithes, so Shabbat establishes the requirement to recite kiddush. Generally, one may eat untithed produce in a casual, incidental manner. On Shabbat, however, the strictures of a regular, set meal apply even to casual eating. Consequently, on Shabbat it is entirely prohibited to eat produce from which the appropriate dues and tithes have not yet been separated. Similarly, Shabbat automatically initiates the requirement to recite kiddush, and it is prohibited to eat until one does so. This halakha indicates that whatever one eats at this stage is considered part of his Shabbat meal, even if he does not remove the table and bring it back.