Berachos (27b): It is related that a certain disciple came before R. Joshua and asked him, Is the evening Tefillah compulsory or optional? He replied: It is optional. He then presented himself before Rabban Gamaliel and asked him: Is the evening Tefillah compulsory or optional? He replied: It is compulsory. But, he said, did not R. Joshua tell me that it is optional? He said: Wait till the champions enter the Beth ha-Midrash. When the champions came in, someone rose and inquired, Is the evening Tefillah compulsory or optional? Rabban Gamaliel replied: It is compulsory. Said Rabban Gamaliel to the Sages: Is there anyone who disputes this? R. Joshua replied to him: No. He said to him: Did they not report you to me as saying that it is optional? He then went on: Joshua, stand up and let them testify against you! R. Joshua stood up and said: Were I alive and he [the witness] dead, the living could contradict the dead. But now that he is alive and I am alive, how can the living contradict the living?