The locus classicus is the text everyone quotes to prove they're right.
A famous (okay, not entirely unknown) one occurred in Victor Hugo's poem Quant à Paris, ton poing l'étreine, which was seized upon by those who were dismayed by Haussmann's plans to make the city of Paris more rational and orderly (which involved knocking down quite a lot of the old bits). Hugo described:
...plus de rues Anarchiques...plus de caprice...
...and the protesters, who rather liked things anarchic and capricious, seized upon this as authority to support their point of view.
(No, of course it wasn't proof of anything, but it was seized upon as a valid argument by plenty of people so it's still a locus classicus.)
As the phrase suggests, the idea of a locus classicus was first applied to Latin and Greek texts, but nowadays it's applied more widely. A host of obvious examples can be found in religious texts: Thou shalt not kill, for instance.
As I said, nothing has to be proved by a locus classicus. It just has to be written down (or, at the least, attributed to somebody) and then cited quite often as an authority.
I just wish that last one was cited a whole lot more.
Phrase To Use Today: a locus classicus. But upon what authority does your example rest?