Is it on-topic to ask on English language & Usage:
Who first said [insert quote]?
No. While ELU can help you find the first usage of a word and how it has changed over time, finding the first utterance of a quote is likely to be closed as General Reference or Off Topic. The difference is that word etymology can be analyzed and explained (though note it's not always on-topic either!) while a quote will generate a short answer like "so-and-so said it first". In addition, the source of a quote isn't really asking something about how the language itself is being used.
For example, https://english.stackexchange.com/questions/63421/need-help-finding-source-of-quote was closed as off topic.
'Who first said X?' questions and their similar etymology questions are sometimes on-topic sometimes not. It actually depends on the answer.
If the answer is in an easily accessible place (like etymonline or possibly OED which keeps lists of earliest references), the the question is closable as General Reference. That is, you probably could have done minimal google research to find your answer.
A difficulty with these kinds of questions is that sometimes it is just not knowable. "Was it Oscar Wilde or George Bernard Shaw? My high school teacher said.." There might be some articles out there where the research was done, but it might be too arguable.
You should definitely do the research ahead of time, but if you can't find the answer with OED or etymonline, definitely ELU is the place to go for a mildly on-topic answer.
Such a question would be more on-topic for a site like Literature.SE which was unfortunately obsoleted.
But if your question is something like "Who first said 'The nut doesn't fall far from the tree'?" it might be quickly closed for not having a good answer.