In Genesis 17, it is written,
24 Abraham was ninety-nine years old when he was circumcised in the flesh of his foreskin. 26 That very same day Abraham was circumcised, and his son Ishmael; NKJV, ©1982
כד וְאַבְרָהָם בֶּן־תִּשְׁעִים וָתֵשַׁע שָׁנָה בְּהִמֹּלוֹ בְּשַׂר עָרְלָתוֹ כו בְּעֶצֶם הַיּוֹם הַזֶּה נִמּוֹל אַבְרָהָם וְיִשְׁמָעֵאל בְּנוֹ
Does the scripture provide any clues regarding who circumcised Abraham? Also, are there any clues in the words בְּהִמֹּלוֹ (behimolo) or נִמּוֹל (nimol)?1
1 Binyan nifʿal can be understood as passive voice (e.g., he was circumcised) or reflexive (e.g., he circumcised himself). See Pratico and Van Pelt, p. 125, § 12.7, 1: “The Niphal stem is used to express simple action with either a passive or reflexive voice.”
Pratico, Gary D.; Van Pelt, Miles V. Basics of Biblical Hebrew Grammar. 2nd ed. Grand Rapids: Zondervan, 2007.