וְאָכַלְתָּ וְשָׂבָעְתָּ וּבֵרַכְתָּ אֶת יְהוָה אֱלֹהֶיךָ עַל הָאָרֶץ הַטֹּבָה אֲשֶׁר נָתַן לָךְ.
And thou shalt eat and be satisfied, and bless the LORD thy God for the good land which He hath given thee.
אמר רב יהודה מנין לברכת המזון לאחריה מן התורה שנאמר (דברים ח, י) ואכלת ושבעת וברכת.
Rav Yehudah said: Where do we find the blessings after meals in the Torah? As it is said (Deut. 8:10) 'you shall eat, and you shall be satisfied, and bless'.
נשים ועבדים וקטנים, אין מזמנין עליהם. עד כמה מזמנין, עד כזית. רבי יהודה אומר, עד כביצה.
Women, slaves, and children - one cannot convene with them. How much is needed [to be eaten in order] to convene? At least an olive['s volume]. Rabbi Yehuda says at least an egg['s volume].
...אין זו בכלל ברכה, אלא אם כן עשאה לחם.
Rashi on Brachot 44a
... This is not blessing in general, but rather in the case of bread.
...מהדרינן אר"מ דסתם מתני' ר"מ...
... We tried if possible [to base ourselves on the authority of] R. Meir because, where a Mishnah is stated anonymously [its author is] R. Meir...
...ר' מאיר סבר ואכלת זו אכילה ושבעת זו שתיה ואכילה בכזית ורבי יהודה סבר ואכלת ושבעת אכילה שיש בה שביעה ואיזו זו כביצה.
... Rabbi Meir holds that 'and you shall eat' refers to eating and 'you shall be satisfied' refers to drinking, and [the standard of] eating is an olive['s volume]. And Rabbi Yehuda holds 'and you shall eat and you shall be satisfied' refers to eating [an amount] which satisfies, and this is an egg['s volume].
...כגון שאכל שיעורא דרבנן דאתי דרבנן ומפיק דרבנן...
... For example, if he ate a quantity for which he is only rabbinically bound [to say grace], one who is rabbinically bound may perform the duty for one who is rabbinically bound.
דרש רב עוירא זמנין אמר לה משמיה דר' אמי וזמנין אמר לה משמיה דר' אסי אמרו מלאכי השרת לפני הקב"ה רבש"ע כתוב בתורתך (דברים י, יז) אשר לא ישא פנים ולא יקח שחד והלא אתה נושא פנים לישראל דכתיב (במדבר ו, כו) ישא ה' פניו אליך אמר להם וכי לא אשא פנים לישראל שכתבתי להם בתורה (דברים ח, י) ואכלת ושבעת וברכת את ה' אלהיך והם מדקדקים [על] עצמם עד כזית ועד כביצה: מתני' בעל קרי מהרהר בלבו ואינו מברך לא לפניה ולא לאחריה ועל המזון מברך לאחריו ואינו
R. Avira discoursed — sometimes in the name of R. Ammi, and sometimes in the name of R. Assi — as follows: The ministering angels said before the Holy One, blessed be He: ‘Sovereign of the Universe, it is written in Thy law, Who regardeth not persons nor taketh reward, and dost Thou not regard the person of Israel, as it is written, “The Lord lift up His countenance upon thee?”’ He replied to them: ‘And shall I not lift up My countenance for Israel, seeing that I wrote for them in the Torah, “And thou shalt eat and be satisfied and bless the Lord thy God,” and they are particular [to say the grace] if the quantity is but an olive or an egg.’
מצות עשה מן התורה לברך אחר אכילת מזון שנאמר ואכלת ושבעת וברכת את יי' אלהיך ואינו חייב מן התורה אלא אם כן שבע שנאמר ואכלת ושבעת וברכת ומדברי סופרים אכל אפילו כזית מברך אחריו.
Mishneh Torah Hilkhot Brachot 1:1
It is a positive mitzvah from the Torah to bless [God] after eating satisfying food, as it is said: "When you have eaten and are satiated, you shall bless God, your Lord." The Torah itself requires a person to recite grace only when he eats to the point of satiation, as implied by the above verse, "When you have eaten and are satiated, you shall bless...." The Sages, however, ordained that one should recite grace after eating [an amount of bread equal to] the size of an olive.
If one ate bread and does not know whether or not he made the blessings for after a meal afterwards, he is required to make the blessings in view of the doubt involved, since one is obliged to make these blessings according to Torah law.
The amount of bread one must eat in order to be required to make the blessings for after a meal over it is the bulk of an olive.
If some of the members of a group who ate together ate a sufficiently large quantity to be satisfied and others merely ate an olive’s bulk of bread, then, if they all know how to say the blessings for after a meal the mitzvah is best performed if one of those who ate sufficient to be satisfied makes his blessings serve for the others to fulfil their obligation. However, if they do not all know how to say the blessings, then, if necessary, one of those who merely ate an olive’s bulk may make his blessings serve for the fulfilment of the obligation even of those who ate sufficient to be satisfied.
Gloss: There are authorities who say that one is not obliged by Torah law to make the blessings for after the meal if he did not drink and has an appetite to drink. It is desirable to take care initially when some of the diners drank and some did not that one of those who drank should say the blessings.
Mishnah Berurah 197:25
This is because they are of the opinion that when Rabbi Meir expounds in the Gemara that the word ve-achalta (and you will eat) refers to eating and the word ve-savata (and you will be satisfied) refers to drinking, what is meant is that if one drank after he ate he is then obliged to bless by Torah law. If he did not drink, however, he is not obliged to bless by Torah law, but only by Rabbinical law.
Mishnah Berurah 184:15
‘Since one is obligated to make these blessings after a meal according to Torah law.’ This only applies if one became satisfied from the meal, since if one did not become satisfied he is merely obliged to make the blessings according to Rabbinical law, in the opinion of the majority of Poskim. Nevertheless, if one is a person who fears Heaven, then, even when he merely ate an olive’s bulk of bread and is in doubt as to whether or not he made the blessings for after the meal afterwards, it is proper for him to wash his hands, make the blessing Ha-Motzi and eat an olive’s bulk of bread, so that he will be able to make the blessings for after a meal and thus discharge any obligation he may have as regards the previous meal.
Mishnah Berurah 184:22
This is the amount for which one is obligated to make the blessings by Rabbinical law, but insofar as Torah law is concerned one is not obligated to make the blessings for after a meal unless he actually ate a quantity which satisfies, as may be seen from what is stated, “And you shall eat and be satisfied and bless.”
It is implied by the Sefer Ha-Chinuch [in the weekly portion Eykev] that the amount which satisfies is not the same for everybody, but each person is aware how much food he requires to be satisfied. If one is accustomed at all times to eat only what is sufficient for his subsistence, this is also considered satisfaction.