The Torah mentions, after the story of Yaakov getting injured in Parshas Vayishlach, that we do not eat the Gid Hanashe for this reason.
The Meforshim seem to concur that this mitzvah was included in Moshe's kabbalas hatorah on Har Sinai, but the Torah never specifically mentions that Moshe ever told anyone about it like he did all the other mitzvos. Did Moshe give it over and the Torah just neglects to mention it, or did he for some reason not need to tell the rest of the nation?
The Mishnah (Chullin 100b) records a debate between R' Yehudah and the Sages about this. According to the majority view, the verse that records the prohibition was indeed inserted there at the time of the Giving of the Torah - i.e., this is Moshe (speaking in Hashem's name, of course) telling the people: "Therefore, [from now on,] the Children of Israel are not to eat the gid hanasheh..."
Elsewhere (Sanhedrin 59a-b) the Gemara points out that according to R' Yehudah, gid hanasheh is the only example of a mitzvah that was "given to the Noachides and not repeated at Sinai," and which is therefore binding only on Jews. According to this view, the point is that the Torah didn't need to tell us that Moshe repeated it to the people, because it is obvious that "there is nothing which is prohibited to non-Jews and permissible to Jews" (see also Tosafos there ד"ה ליכא מידעם).
