Parashas Shemos
וַתִּקַּ֨ח צִפֹּרָ֜ה צֹ֗ר וַתִּכְרֹת֙ אֶת־עׇרְלַ֣ת בְּנָ֔הּ וַתַּגַּ֖ע לְרַגְלָ֑יו וַתֹּ֕אמֶר כִּ֧י חֲתַן־דָּמִ֛ים אַתָּ֖ה לִֽי׃
iMeaning of verses uncertain. So Zipporah took a flint and cut off her son’s foreskin, and touched his legs with it, saying, “You are truly a bridegroom of blood to me!”
Torah Temimah points to Nedarim 32a / Avodah Zarah 27a (verified).
ותקח צפורה צור — מלמד שהאשה כשרה למול.
English: “And Tzipporah took a flint” — this teaches that a woman is valid to perform circumcision.
Custom
Chazal derive from Tzipporah’s act that a woman is qualified to circumcise.