Reference for Avodah Zarah 36:6

ורמינהי [הולכין] לאיצטדינין מותר מפני שצווח ומציל ולכרקום מותר מפני ישוב מדינה ובלבד שלא יתחשב עמהם ואם נתחשב עמהם אסור קשיא איצטדינין אאיצטדינין קשיא כרקום אכרקום

They contrasted the above with the following: It is permitted to go to stadiums, because by shouting one may save [the victim]. [It is also permitted to go] to a siege camp because of those who dwell there, as long as he does not conspire [with the Romans]. But if it is for the purpose of conspiring, it is forbidden. There is thus a contradiction between [the laws relating to]stadiums as well as between [those relating to] siege camps!

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