It is clear that Abraham understood the Torah, and that he waited to enact his bris until commanded, but who performed it? Did he do it himself, or was someone else instructed to do it, to serve in the sealing of his covenant?
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Pirkei D'Rebbi Eliezer 29 says that Shem Ben Noach performed Avraham's bris.
The Medrash Rabba Bereishis 49:2 says that Hashem held Avraham's hand and helped him perform the bris.
The verse in Bereishis 17:24 supports that it was performed by someone else, as it uses the passive language בהמולו, when he was circumcised, and Rashi there points out that this is הפעל, acted upon.
This website recounts the medrash that says that
The Midrash Rabbah Lech Lacha 9: "א"ר לוי מל אברהם אין כתיב כאן אלא נימול בדק את עצמו ומצא עצמו מהול "
Reb Levi said that the Torah said nimol Avraham opposed to maol Avrham,meaning Avraham checked and saw that he had a milah already.