So every table I've been at accepts the common idea that whenever you cast a spell there is a non-specified, but observable (unhideable, except w/certain feats) manifestation that reveals you have cast a spell.
Where is the source for that? I'll take either a CRB page number or PRD link.
NOTE: I'm not questioning this or looking to start a thing, I just realized I had been going with the flow on this and never actually seen a written rule about it.
Thanks!