I think you are referring to this statement:
The Pharisees made the claim that believers could not remain uncircumcised and also "keep the Law of Moses" even though only 8th day circumcision is mandated by Torah.
Where in the Torah does God mandate believers to become circumcised as adults?
Leviticus 12 where it says a Gentile who wants to keep the Passover must be circumcised. This was basically the process of a person immigrating to Israel and becoming a naturalized citizen in ancient Israel.