Commentary for Avodah Zarah 53:8

איתמר מנין למילה בעובד כוכבים שהיא פסולה דרו בר פפא משמיה דרב אמר ואתה את בריתי תשמור ורבי יוחנן המול ימול

It has been stated: From where do we know that circumcision performed by a non-Jew is invalid? Daru b. Papa said in the name of Rav: [From the words,] “And as for you, you shall keep my covenant.” While R. Yohanan: “Circumcise, you shall surely circumcise” [which he reads as if it says “he who is circumcised shall circumcise.”

Daf Shevui to Avodah Zarah