From Superman (1939) #19, page 35
Transcript:
Narration: Later... Clark enters the Planet building, accompanied by his canine friend....
Clark Kent: I wonder why they put up signs like that? Dogs can't read!
Sign: No Dogs Allowed
TBH I think this is a joke in the original and would fall under R3 but it's ambiguous so I'll leave it up. I haven't really been moderating this community much but I plan to do a bit more, since that seems to be a point of confusion.