Rav Abraham Palachi (son of the famous Rav Haim Palachi of Izmir, Turkey) was once asked why the Sages did not enact a Beracha to be recited before eating bread on Shabbat. On Pesah, for example, when we have an obligation eat Masa, we recite the Beracha "Asher Kideshanu Be’misvotav Ve’sivanu Al Achilat Masa" before we eat Masa. We would therefore expect that, on Shabbat, when we are required to eat bread, we should likewise recite a Beracha "Asher Kideshanu Be’misvotav Ve’sivanu Le’echol Pat Be’Shabbat," or something to that effect. Yet, no such Beracha was enacted. Rav Abraham was asked to explain the difference between eating Masa on Pesah – which is preceded by a Beracha – and eating bread on Shabbat, which is not introduced with a Beracha.
Rav Abraham responded, surprisingly enough, by denying the entire basis of the question. He claimed that there is no obligation at all to eat bread on Shabbat. Halacha forbids fasting on Shabbat, but there is no strict obligation to eat bread.
The later Halachic authorities strongly dispute Rav Abraham’s response. The requirement to eat bread on Shabbat is well-documented in the writings of the Rishonim (Medieval scholars), and in fact appears explicitly in the commentary of the Rashba (Rabbi Shelomo Ben Aderet of Barcelona, 1235-1310). The Shulhan Aruch also codifies this obligation. What makes Rav Abraham’s comments especially baffling is the fact that his father, Rav Haim Palachi, writes in this Kaf Ha’haim that eating bread on Shabbat constitutes a Torah obligation.
Thus, notwithstanding the question of why we do not recite a Beracha before eating bread on Shabbat, the clear consensus among the Halachic authorities is that one is obligated to eat at least two meals with bread on Shabbat. When it comes to Se’uda Shelishit, the third meal, it is certainly preferable to eat bread, but many authorities rule that it suffices to eat Mezonot food or fruit for Se’uda Shelishit. For the first two Shabbat meals, however, one is strictly obligated to eat bread. It should be noted that one must eat at these meals at least 1 oz. of bread, and preferably 2 oz. Many people take just a few small bites of Halla at the Shabbat meal, and do not ensure to eat the proper quantity. They not only fail to fulfill the obligation of eating bread on Shabbat – which, as we saw, is considered by some as a Torah obligation – but they also end up eating less than the amount requiring Birkat Hamazon. Thus, when they recite Birkat Hamazon, all the Berachot are considered "Berachot Le’batala" (Berachot recited in vain, which is a grave transgression).
Since eating the first two bread meals on Shabbat is a Halachic obligation, one who forgets "Reseh Ve’hahalisenu" in Birkat Hamazon after one of these two meals must repeat Birkat Hamazon. Halacha establishes that whenever a meal is obligatory, one must repeat Birkat Hamazon if he forgets to say the required inserted text, and this rule would apply to the first two meals of Shabbat. The Gemara in Masechet Berachot (49) writes that if a person forgot to recite "Reseh" but remembered just after he recited "Boneh Yerushalayim," before beginning the next Beracha, then he recites a special Beracha, "Baruch Ata…Asher Natan Shabbatot …Baruch Ata Hashem Mekadesh Ha’Shabbat." (The full text appears in many Siddurim.) If, however, one did not realize his mistake until he began the next Beracha, then he must return to the beginning of Birkat Hamazon.
This applies only to the first two Shabbat meals. When it comes to Se’uda Shelishit, which, as mentioned, does not require bread according to some authorities, one who forgets "Reseh" does not repeat Birkat Hamazon. Similarly, Tosafot write that if a person eats a fourth meal on Shabbat and forgets "Reseh" in Birkat Hamazon after that meal, he does not repeat Birkat Hamazon, since that meal was not required by Halacha.
(See also "If one Forgot to Add Ritze Ve’hachalitzenu in Birkat Hamazon on Shabbat" dated 5/29/06, and "One Who Forgot to Add Reseh in Birkat Hamazon on Shabbat" dated 7/15/09.)
Summary: One is obligated to eat bread with at least two meals on Shabbat, and preferably with the third meal, as well. One who forgot to add "Reseh" in Birkat Hamazon after one of the first two Shabbat meals must repeat Birkat Hamazon; he does not repeat Birkat Hamazon if this occurred at another Shabbat meal.
