As we have learned (see above, daf, or page 90), the gid ha-nasheh – an animal’s sciatic nerve – is prohibited based on the passage in Sefer Bereishit (32:33). Following his wrestling with an angel, Ya’akov limps back towards his camp and the Torah concludes “Therefore the children of Israel eat not the sinew of the thigh-vein which is upon the hollow of the thigh, unto this day; because he touched the hollow of Jacob’s thigh, even in the sinew of the thigh-vein.”
The Mishnah on today’s daf limits the prohibition to kosher animals; Rabbi Yehudah disagrees, arguing that before the Torah was given – when Ya’akov’s children were still allowed to eat unclean animals – they refrained from eating the gid ha-nasheh. The response quoted in the Mishnah is that the commandment prohibiting the gid ha-nasheh was given at Mount Sinai, where it was then placed in context in Sefer Bereishit. It does not indicate, however, that Ya’akov’s children kept this tradition prior to receiving the Torah.
The general principle regarding Biblical commandments is that they are incumbent upon the Jewish People because they were given as part of the Torah, and not because they may have been related as part of the earlier Biblical narrative. For example, the Rambam explains that the mitzvah of brit milah – circumcision – is not kept because Avraham was commanded to circumcise himself and the members of his family, rather because the Torah commands us to circumcise our children in the manner that Avraham was commanded.
On tomorrow’s daf the Gemara relates that the argument presented by the Hakhamim to Rabbi Yehudah is that the passage in Sefer Bereishit relates to “Benei Yisrael” – the Children of Israel – and that the Jewish people only formally became “Benei Yisrael” after their acceptance of the Torah at Mount Sinai. While Rashi explains that the tradition of refraining from eating the gid ha-nasheh was not kept until that time, it can be understood to mean that the commandment that the Jewish People keep today is qualitatively different from the traditions that were kept by Ya’akov’s children and family prior to accepting the Torah.