Why Gid-Haneshe is counted as a Mitzvah if it wasn't commanded?

Question

עַל־כֵּן לֹא־יֹאכְלוּ בְנֵי־יִשְׂרָאֵל אֶת־גִּיד הַנָּשֶׁה אֲשֶׁר עַל־כַּף הַיָּרֵךְ עַד הַיּוֹם הַזֶּה כִּי נָגַע בְּכַף־יֶרֶךְ יַעֲקֹב בְּגִיד הַנָּשֶׁה׃

That is why the children of Israel to this day do not eat the thigh muscle that is on the socket of the hip, since Jacob’s hip socket was wrenched at the thigh muscle. (Gen 32.33)

THe text of the Torah explicitly says it's a Minhag, sons of Israel don't eat the tendon only customary, in memory of Yaakov's fight, and not because of G-d's commandment.

Rambam in Melachim 9.1 summarizes it very weirdly IMHO, mixing Mitzvos and customs:

...בָּא אַבְרָהָם וְנִצְטַוָּה יֶתֶר עַל אֵלּוּ בְּמִילָה. וְהוּא הִתְפַּלֵּל שַׁחֲרִית. וַיִּצְחָק הִפְרִישׁ מַעֲשֵׂר וְהוֹסִיף תְּפִלָּה אַחֶרֶת לִפְנוֹת הַיּוֹם. וְיַעֲקֹב הוֹסִיף גִּיד הַנָּשֶׁה וְהִתְפַּלֵּל עַרְבִית.
... With Abraham, circumcision was also commanded and he prayed Shacharis (the Morning Prayer). Isaac separated out a tithe and added another prayer in the afternoon and, with Jacob, the prohibition against eating the sciatic nerve was added, as was the Maariv (Evening) Prayer.

We can see that some (circumcision) are explicit commandments, while others (Teffilot) are just customs, they are not counted in the list of the 613 Mitzvos. He does count it as a Biblical prohibition (לא תעשה קפ״ג).

How did Gid-Haneshe become a Mitzvah?

Answer 1

See Chulin 101b

Rabbi JUDAH ARGUED, WAS NOT THE SCIATIC NERVE FORBIDDEN FROM THE TIME OF THE SONS OF JACOB? etc. It was taught: [The Rabbis] said to Rabbi Judah: Does it say [in the Torah], 'Therefore the children of Jacob eat not'? Surely it says: Therefore the children of Israel eat not. Now they were first styled the children of Israel only at [the giving of the law at] Sinai; therefore [we must say that] the law [of the sciatic nerve] was given at Sinai, but was written in its present place to indicate the reason why it was prohibited. Raba raised an objection against this. It is written: And the sons of Israel carried Jacob their father!(Genesis 46.5) - That was after the incident. Rav Aha the son of Raba said to Rav Ashi: Then it should be prohibited from that time onwards, should it not? (1) - He replied: Was the Torah given at various times? And that time was neither the time of the incident nor the time of the giving of the Law.

For Rabanan two times are possible for giving the mitsva, the time of the event and the time of Matan Tora. The second one is the right because at this time the name"Bene Israel" was used.

For Rabbi Yehuda, maybe that it's different,at the first glance, but the Rambam writes on the Mishna

...ושים לבך על העיקר הגדול הנכלל במשנה הזאת והוא מה שאמר מסיני נאסר לפי שאתה הראית לדעת שכל מה שאנו מרחיקים או עושים היום אין אנו עושין אלא במצות הקב"ה ע"י משה רבינו ע"ה לא שהקב"ה אמר זה לנביאים שלפניו כגון זה שאין אנו אוכלין אבמ"ה אינו מפני שהקב"ה אסרו [אותו] לנח אלא לפי שמשה אסר עלינו אבמ"ה במה שצוה בסיני שיתקיים איסור אבמ"ה וכמו כן אין אנו מלין מפני שאברהם אבינו ע"ה מל עצמו ואנשי ביתו אלא מפני שהקב"ה צוה אותנו ע"י משה רבינו שנמול כמו שמל אברהם אבינו ע"ה וכן גיד הנשה אין אנו הולכים אחר איסור יעקב אבינו אלא מצות משה רבינו ע"ה הלא תראה מה שאמרו תרי"ג מצות נאמרו לו למשה מסיני וכל אלו מכלל המצות

And he seems to explain that for every people, including Rabbi Yehuda, all Mitsvot were given at Matan Tora, even those that were written in Bereshit. But the Chatam Sofer in Chiddushim on the Daf says that the words of the Rambam don't apply for Rabbi Yehuda. For Rabanan all Mitsvot were given in Sinai, including the 3 mitsvot written in Sefer Bereshit, but for Rabbi Yehuda the mitsva of Gid Hanashe was already given to Yaacov. But even for Rabanan, Rambam wrote in Hilchot Melachim 9.1 that Yaacov Avinu added Mitsvat Gid Hanashe. So, there is an apparent contradiction in the line of Rabanan, according to the understanding of Rambam. The Lechem Mishne asks this and concludes "יעקב הנהיג עצמו וביתו לאסור גיד הנשה" but says that he doesn't know from where the Rambam learns this. So, for Rabanan at the time of Yaacov it was a minhag, or more exactly a kind of Noachid Mitsva, in Erets Israel only, but from Matan Tora it became a Mitsva Deorayita. For Rabbi Yehuda, it was already a Mitsva from Yaacov. See in the Mefarshim on the Rambam additional explanations. The Bigde Yesha seems to say that the Machloket between Rabbi Yehuda and Rabanan is if the Mitsva given on Sinai is an exact copy of the Mitsva of Yaacov or not.

(1) Sefer Hayashar from Rabenu Tam

והמקשה טועה שסובר שמשעה שנקראו ישראל יחול איסור גיד הנשה ובשביל כך הוא מקשה מאותו מעשה שנקראו ישראל ליתסר. ומשני וכי תורה פעמ' כו'. אותה שעה שקרא המלאך ישראל לא שעת מעשה נשיות גיד הוה שבשביל כך נאסר ולא שעת מתן תורה הוה. ‏