Where else do we find, like the sifsei chachomim writes (Bereshit 45:4), that bris milah is a bris achvah - a covenant of brotherhood?
Restating the question
To set this up, let's start with the context: This is the absolute climax of the story of Yosef and his brothers. In the v. 3, Yosef reveals that he was not just the viceroy of Egypt, but Yosef, the brother they'd cast into a pit and then sold down the river so many years ago. The brothers, shocked, are unable to answer, so Yosef asks them (v. 4) to "come close," and they do. Rashi comments:
רָאָה אוֹתָם נְסוֹגִים לְאָחוֹר, אָמַר עַכְשָׁו אַחַי נִכְלָמִים, קָרָא לָהֶם בְּלָשׁוֹן רַכָּה וְתַחֲנוּנִים, וְהֶרְאָה לָהֶם שֶׁהוּא מָהוּל:
He saw them drawing backwards. He said,“Now my brothers are embarrassed”. He called them tenderly and pleadingly and showed them that he was circumcised.
Ikar Siftei Chachamim1 elaborates on Rashi's comment:
... ולקרבם אליו אמר אני יוסף אחיכם והראה להם שהוא מהול כי הוא ברית אחוה בישראל אשר לא תופר לעד
... And to bring them close to him, he said "I am Yosef your brother," and he showed them that he was circumcised, for that is a covenant of brotherhood that shall never be nullified.2
So, where does this surprising characterization of circumcision - "a covenant of brotherhood that shall never be nullified" - come from?
I believe that the Ikar Siftei Chachamim is making a complex allusion to the prophecies of Zecharia.
First of all, the unusual collection of words ברית, אחוה, and הפרה - covenant, brotherhood, and nullification - all show up near each other in Zecharia 11:10-143:
וָאֶקַּ֤ח אֶת־מַקְלִי֙ אֶת־נֹ֔עַם וָאֶגְדַּ֖ע אֹת֑וֹ לְהָפֵיר֙ אֶת־בְּרִיתִ֔י אֲשֶׁ֥ר כָּרַ֖תִּי אֶת־כָּל־הָעַמִּֽים׃ וַתֻּפַ֖ר בַּיּ֣וֹם הַה֑וּא וַיֵּדְע֨וּ כֵ֜ן עֲנִיֵּ֤י הַצֹּאן֙ הַשֹּׁמְרִ֣ים אֹתִ֔י כִּ֥י דְבַר־יְהוָ֖ה הֽוּא׃ וָאֹמַ֣ר אֲלֵיהֶ֗ם אִם־ט֧וֹב בְּעֵינֵיכֶ֛ם הָב֥וּ שְׂכָרִ֖י וְאִם־לֹ֣א ׀ חֲדָ֑לוּ וַיִּשְׁקְל֥וּ אֶת־שְׂכָרִ֖י שְׁלֹשִׁ֥ים כָּֽסֶף׃ וַיֹּ֨אמֶר יְהוָ֜ה אֵלַ֗י הַשְׁלִיכֵ֙הוּ֙ אֶל־הַיּוֹצֵ֔ר אֶ֣דֶר הַיְקָ֔ר אֲשֶׁ֥ר יָקַ֖רְתִּי מֵֽעֲלֵיהֶ֑ם וָֽאֶקְחָה֙ שְׁלֹשִׁ֣ים הַכֶּ֔סֶף וָאַשְׁלִ֥יךְ אֹת֛וֹ בֵּ֥ית יְהוָ֖ה אֶל־הַיּוֹצֵֽר׃ וָֽאֶגְדַּע֙ אֶת־מַקְלִ֣י הַשֵּׁנִ֔י אֵ֖ת הַחֹֽבְלִ֑ים לְהָפֵר֙ אֶת־הָֽאַחֲוָ֔ה בֵּ֥ין יְהוּדָ֖ה וּבֵ֥ין יִשְׂרָאֵֽל׃
Taking my staff Favor, I cleft it in two, so as to annul the covenant I had made with all the peoples; and when it was annulled that day, the same poor men of the sheep who watched me realized that it was a message from the LORD. Then I said to them, “If you are satisfied, pay me my wages; if not, don’t.” So they weighed out my wages, thirty shekels of silver— the noble sum that I was worth in their estimation. The LORD said to me, “Deposit it in the treasury.” And I took the thirty shekels and deposited it in the treasury in the House of the LORD. Then I cleft in two my second staff, Unity, in order to annul the brotherhood between Judah and Israel.
The classical commentators offer various interpretations of this prophecy. Both Rashi and Radak say that it's a prophetic retelling of the history of the destruction of the First Temple and the dispersal that accompanied it, first of the northern kingdom of Israel, then of the southern kingdom of Judea. With the breaking of the first staff - allowing Aram to dominate and exile Israel - God nullified a covenant He'd made with the surrounding nations forbidding them to harm the Jews as long as the Jews kept the Torah. With the breaking of the second staff - the destruction of the Temple and exile of Judea - God nullified a brotherhood between Israel and Judea that had rested on a shared interest in worshiping idols.
This is sad, anti-brotherly, and anti-covenental, and it has nothing to do with circumcision. Why did Ikar Siftei Chachamim allude to it?
I think we can find the answer if we turn back a couple of chapters in Zecharia, to a much happier prophecy (9:9-11):
גִּילִ֨י מְאֹ֜ד בַּת־צִיּ֗וֹן הָרִ֙יעִי֙ בַּ֣ת יְרוּשָׁלִַ֔ם הִנֵּ֤ה מַלְכֵּךְ֙ יָ֣בוֹא לָ֔ךְ צַדִּ֥יק וְנוֹשָׁ֖ע ה֑וּא עָנִי֙ וְרֹכֵ֣ב עַל־חֲמ֔וֹר וְעַל־עַ֖יִר בֶּן־אֲתֹנֽוֹת׃ וְהִכְרַתִּי־רֶ֣כֶב מֵאֶפְרַ֗יִם וְסוּס֙ מִיר֣וּשָׁלִַ֔ם וְנִכְרְתָה֙ קֶ֣שֶׁת מִלְחָמָ֔ה וְדִבֶּ֥ר שָׁל֖וֹם לַגּוֹיִ֑ם וּמָשְׁלוֹ֙ מִיָּ֣ם עַד־יָ֔ם וּמִנָּהָ֖ר עַד־אַפְסֵי־אָֽרֶץ׃ גַּם־אַ֣תְּ בְּדַם־בְּרִיתֵ֗ךְ שִׁלַּ֤חְתִּי אֲסִירַ֙יִךְ֙ מִבּ֔וֹר אֵ֥ין מַ֖יִם בּֽוֹ׃
Rejoice greatly, Fair Zion; Raise a shout, Fair Jerusalem! Lo, your king is coming to you. He is victorious, triumphant, Yet humble, riding on an ass, On a donkey foaled by a she-ass. He shall banish chariots from Ephraim And horses from Jerusalem; The warrior’s bow shall be banished. He shall call on the nations to surrender, And his rule shall extend from sea to sea And from ocean to land’s end. You, for your part, have released Your prisoners from the dry pit, For the sake of the blood of your covenant,
Again, the classical commentators offer various approaches to this prophecy. Rashi and Radak say that it speaks of the ultimate Messianic era. In those days, God will re-unite Israel (Ephraim) and Judea (Jerusalem) to live together in peace. In response to what merit will God pull Jews out of exile in "the pit with no water (prophecy)"? According to Radak, the merit that gets them out will be that of Berit Mila - the covenant of circumcision, an observance which Jews held onto all through the exile.
Here we have the circumcision that I believe the Ikar Siftei Chachamim is having Yosef refer us to. Yosef had, having been cast into a pit without water, gone into exile, but steadfastly held onto his Jewish identity, exemplified by his circumcision. Where all other bonds holding him and his brothers together had failed, the core circumcision covenant remained, allowing him to establish his identity to his brothers and effect their re-union. So, too, says Zecharia, though all the other covenants that once held the Jewish nation together may have failed, one by one, as long as the core identity expressed in the practice of Mila persists, the basis for the the ultimate re-union and redemption of all the far-flung tribes persists. May God realize this prophecy speedily, in our days.
1. Note that this association is not found at all in the comment here by the original Siftei Chachamim, though it is in the comment of the Ikar Siftei Chachamim. Apparently, the latter is not merely a straight-up summary of the former, and it's not clear who wrote it.
2. Translation and emphasis mine.