- Since the obligation to reciteBircas Hamazon (Grace after Meals) is mandated by Torah-law, a person who ate bread but cannot remember if he recited Bircas Hamazon or not must recite it now because of this doubt (Shulchan Aruch OC 183 with Mishnah Berura). In this situation, some authorities maintain that the person should also recite the fourth blessing of Bircas Hamazon [despite the fact that the fourth blessing was instituted by the Rabbis] (Mishnah Berura), while other authorities maintain that in this case the person omits the fourth blessing (Ben Ish Chai; Kaf Ha-Chaim).
2. However, this rule only applies when the person is sated from the meal, since only then is one’s obligation to recite Bircas Hamazon mandated by the Torah [while if the person ate less than that amount his obligation would only be on the Rabbinic level]. Nevertheless, it is proper for a God-fearing Jew to wash again and eat an olive-sized volume of bread — this way the person will definitely be allowed to recite Bircas Hamazon. (ibid.)
