Why does the Torah omit the word "Es" from the phrase "be'Himolo B'sar Orlaso" regarding the Milah of Avraham, but insert it by Yishmael in the following Pasuk (17:25)?
Rashi (to 17:25): Because whereas Avraham, whose foreskin, due to Tashmish, was softened, and needed only to be cut, that of Yishmael also required Peri'ah (tearing and folding over).
What do we learn from the word "be'Himolo"?
Da'as Zekenim (to 17:26), Yalkut Shimoni (to Nechemyah Sof Perek 9, 1071): Avraham circumcised himself with Hashem's help, as if He held the knife together with him, for Avraham feared to perform the Bris, due to his old age. This is why the verse says, "v'Charos Imo ha'Bris" (Nechemyah 9:8). 1
Avraham fulfilled the entire Torah without being commanded. Why did he delay Milah until he was commanded?
Da'as Zekenim: R. Tam of Orleans answered that it is better to fulfill the Mitzvah when one is commanded. 1 However, Avraham knew that he would not be commanded about Peri'ah. Why did he delay this aspect? R. Shimon of London said that he did not want to wound himself. Rather, he fulfilled the entire Torah only after circumcising. Or, we can say that Peri'ah was physically unnecessary in Avraham's case, as Rashi explained (refer to 17:24:1:1).
Also Moshav Zekenim says so. Why did he fulfill other Mitzvos? He knew that he would not be commanded about them. (Seemingly, unlike Milah (which can only be performed once), there would be no reason to refrain from other Mitzvos. He could do them now, and again after he was commanded! - PF)
QUESTIONS ON RASHI
Rashi writes (to 17:25): "Yishma'el was a child, and his Milah... also required Peri'ah; therefore the Torah adds the word 'Es.'" However, the Gemara (Yevamos 71b) writes that Avraham was not commanded at all regarding Peri'ah?
Tosfos to Yevamos 71b: Avraham voluntarily performed Peri'ah (for those males for whom it was necessary), even though he had not been commanded to so, just as he voluntarily kept all of the Mitzvos (Yoma 28b).
Gur Aryeh #1: The word Peri'ah means "uncovering completely." Rashi does not mean actual Peri'ah as it was later commanded to Bnei Yisrael (tearing and folding back the underlying membrane of the Orlah), but rather ensuring that none of the remaining actual flesh of the Orlah even appeared to cover the Atarah. Avraham's Milah required less careful attention, for it had already become softened and would not appear as such.
Gur Aryeh #2: The Midrash (Bereishis Rabah 57:8) in fact does not mention Peri'ah here. Avraham 's Milah merely needed cutting, whereas Yishma'el's Milah required careful removal of the entire Orlah.