I was wondering - the Torah seems to mention the gid hanashe only in Parshas Vayishlach which is before matan Torah. Doesn't a mitzvah have to be repeated after matan Torah in order for it to be binding for us ledorot? Is it mentioned again and where? There are probably other mitzvos like that where it's not mentioned mefurash after matan Torah. (How about peru urevu?) Thank you.
Dear Sam, The Gemara in Sanhedrin, 59a & b, discusses the very (valid) points that you made. Rebbi Yossi b'Rebbi Chaninah makes the statement: Any Mitzvah that was said to the B'nei No'ach and repeated at Sinai was said to the B'Nei No'ach as well as to the B'nei Yisrael, whereas any Mitzvah that was said to the B'nei No'ach and was not repeated at Har Sinai was said only to the B'nei Yisrael and not to the B'nei No'ach. The Gemara points out that although logic would seem dictate the opposite, Rebbi Yossi's statement is based on textual and logical proofs.
The Gemara goes on to qualify Rebbi Yossi b'Rebbi Chanina's statement: "And there is only one single case which was not repeated at Har sinai: Gid ha'Nasheh, according to the opinion of Rebbi Yehudah" (who holds that Gid ha'Nasheh was said to the B'nei No'ach before the Torah was given. According to the Rabbanan Gid ha'Nasheh was actually said at Har Sinai together with all the other Mitzvos. It is written in Parshas Va'yishlach only because, for obvious reasons, that is the appropriate place to insert it.
The Gemara asks that the Mitzvah of "Pru u'Revu" was repeated at Har Sinai (when the Hash-m said to Moshe: "Go and say to them, Go and return to your tents (i.e. to your wives) ; but you (Moshe) remain with Me", yet it pertains to Yisrael only, and not to the B'nei No'ach.
The Passuk at Har Sinai, replies the Gemara was not repeated at Har Sinai for the sake of repetition, but rather to teach us a specific lesson (which the Gemara goes on to specify).
Thank you for your interest, feel free to share with us any future questions Be'Virkas Kol Tuv Eliezer Chrysler