A voice of reason: It’s a fairly good Makor. It’s talking about paying a tax to the non-Jewish king’s army. All Yeshivah students (I believe the Lashon is Tzurva MeiRabannan, not Talmid Chacham) are Pattur from the tax. You can argue about whether or not a tax is equivalent to fighting in the army. But the source is a good Makor.
Simcha: I thought so. I looked it up and it’s actually 7b-8a. Sorry.