(בראשית כד ב)
שים נא ידך תחת ירכי
ופירש רש"י תחת ירכי לְפִי שֶׁהַנִּשְׁבָּע צָרִיךְ שֶׁיִּטוֹל בְּיָדוֹ חֵפֶץ שֶׁל מִצְוָה, כְּגוֹן סֵפֶר תּוֹרָה, אוֹ תְּפִילִין (שבועות ל"ח), וְהַמִּילָה הָיְתָה מִצְוָה רִאשׁוֹנָה לוֹ, וּבָאָה לוֹ עַל יְדֵי צַעַר וְהָיְתָה חֲבִיבָה עָלָיו וּנְטִלָהּ:
תחת ירכי UNDER MY THIGH — because whoever takes an oath must take in his hand some sacred object. such as a Scroll of the Law or Tefillin (Shevuot 38b). As circumcision was the first commandment given to him and became his only through much pain it was consequently dear to him and therefore he selected this as the object upon which to take the oath (Genesis Rabbah 59:8).
Question: Avraham avinu was commanded by Hashem "לֶךְ־לְךָ֛ מֵאַרְצְךָ֥ וּמִמּֽוֹלַדְתְּךָ֖ וּמִבֵּ֣ית אָבִ֑יךָ אֶל־הָאָ֖רֶץ אֲשֶׁ֥ר אַרְאֶֽךָּ׃
“Go forth from your native land and from your father’s house to the land that I will show you."
This was before Hashem gave Avraham the Mitzvah of milah
Why didn't Avraham consider that the first commandment given to him?