Continuing the discussion of minhag ha-makom, the status of local custom in Jewish law, the Gemara brings a series of examples of traditions that are not requirements according to halakha and the reactions of the Sages to them. Some examples:
The people of Hozai (an area in Babylon near the Persian Gulf that was far away from the main Jewish community) used to separate halla for the kohanim from rice dough. (The mitzva of halla generally applies only to dough made of wheat, rye, oats, barley and spelt.) Rav Yosef wanted a non-priest to eat the halla in front of them to indicate their error, but Abaye forbade him from doing so, arguing that one should not permit something that has been accepted by the community as being forbidden. Rav Yosef pointed out that according to Rav Hisda, that ruling applied only to Kutai, the Shomronim, a group who had converted and whose commitment to Jewish law was tenuous. Rav Yosef explained that the concern with the Kutai was that they would stop being careful about mitzvot if someone told them that the customs they had been keeping were in error; the same concern applied to Jews living in Hozai.
It is interesting to note that the tradition of treating rice as if it were a real grain is not without precedent. We have learned that, according to Rabbi Yohanan ben Nuri, matza made from rice flour can be used to fulfill the mitzva and is considered hametz if allowed to leaven (see 35a).
Similarly, one may go out with wide shoes that resemble slippers (kurdikison) on Shabbat; however, one does not go out with wide shoes in the city of Birei. And there was an incident involving Yehuda and Hillel, sons of Rabban Gamliel beRabbi, who went out with wide shoes in Birei, and the people of the city denounced them and said: In all our days we have never seen that type of conduct. And Yehuda and Hillel removed their shoes, and gave them to their gentile servants, and did not want to tell the residents of the city: You are permitted to go out with wide shoes on Shabbat.
Rabba bar bar Hana once traveled to Babylon from his home in Israel. He sat down to eat d’ayitra, animal fat along the stomach which was considered permissible in Israel but thought to be forbidden in Babylon. Two of the leading Sages in Babylon, Rav Avira Sava and Rabba the son of Rav Huna, came to visit him while he was eating. He covered the plate so they shouldn’t see what he was eating.
In answer to the Gemara’s question that Rabba bar bar Hana was obligated to accept the local custom, Abaye explains that as a resident of Israel, he was not obligated to accept the Babylonian minhagim. Rav Ashi argued that since he was just visiting and he fully intended to return to Israel, he was not obligated to accept the Babylonian customs.
The Gemara concludes by mentioning that Rabba bar bar Hana himself instructed his children that they could not eat d’ayitra. He told them that he could do so because of the tradition that he had from Rabbi Yohanan, who he had seen eating it, but that they, who had never seen the great Sage eat it, should accept the general practice and refrain.
The Rosh sums up the various stories by ruling that a reliable custom that was instituted and accepted by the local Rabbinic leadership becomes obligatory, and must be kept even if someone finds himself in another place. A lesser tradition that was accepted by the community members on their own does not obligate, and need not be kept if it is done where people will not see you.