Reference for Ketubot 51:9
רבי יהודה אומר אין מעלין לכהונה על פי עד אחד וכו': רשב"ג היינו רבי אליעזר וכי תימא ערער חד איכא בינייהו דרבי אליעזר סבר ערער חד ורשב"ג סבר ערער תרי האמר רבי יוחנן דברי הכל אין ערער פחות משנים
Judah says: One does not raise [a person] to the priesthood on the testimony of one witness, etc… [ Rabbi Shimon ben Gamaliel says in the name of Rabbi Shimon the son of the assistant chief of priests: [a person] to the priesthood through the testimony of one witness.] Shimon b. Gamaliel is the same as R. Eliezer? And if you will say [that] they differ with regard to an objection raised by one person [in] that R. Eliezer holds that an objection [can be accepted from] one [person] and R. Shimon b. Gamaliel holds that an objection [must come from at least] two [people], did not R. Yohanan say: All agree that an objection [must come from] at least two persons?