Parashas Vayeira · Passuk 21:4
And Avraham circumcised his son Yitzchak when he was eight days old, as God had commanded him.
Torah Temimah points to Kiddushin 29a.
TT lemma: וימל אברהם / כאשר צוה אותו.
English: “And Avraham circumcised.” The father is obligated to circumcise his son, as it is written, “And Avraham circumcised his son Yitzchak… as God had commanded him.” The school of Rabbi Yishmael taught: Wherever “command” (tzav) is stated it means urgency, both immediately and for all generations. “As God had commanded him”—him and not her; a woman is not obligated to circumcise her son.
Derash on the verse (Bavli sugya) — not a formal named middah of the 13. The wording “him” and the term “commanded” establish obligation, urgency, and gender distinction.
A father is obligated to circumcise his son; the mitzvah is binding for all generations; a mother is not the primary obligated party.