Rambam writes in perush hamishnayot on Hullin 7:6perush hamishnayot on Hullin 7:6 that the reason we keep certain mitzvot today such as ever min hachai isn't cuz it was commanded to Biney Noach, but rather because it was given over at Sinai. The reason why we keep pre-matan-torah mitzvot such as Milah isn't because it was commanded by god to Avraham avinu, but rather because it was (re)given to Moshe from God to give to Am Yisrael. The first mitzva given to Am Yisrael thru Moshe was hachodesh hazeh lachem. (Also, according to Hachamim in that mishnah, gid hanashe wasn't actually given to Ya'akov but was rather given at sinai and only written in Vayishlach retroactively.) וע"ש