I think the fact that we requre/strongly ask OPs to provide evident research effort prior to asking is sort of an written rule. It's really embedded in the minds of the users that have been using the SE sites. Nevertheless, there is no explicit mention in the standard FAQ about researching and then asking.
We often see closures because users:
- don't search the dictionary
- don't provide their code (gimme teh codez), the user did/didn't write the code: who knows?
- ask about an error that has been treated several times before
- ask for translations without telling what they have tried
and so on. All of the questions where someone has said or wanted to say the famous "What have you tried?" are included here.
I know, lots of users don't even read the FAQ, but besides the fact that rules must be added regardless of that, this one is useful also for justifying actions against questions (i.e. downvoting/closing) that might be "challenged"; I think a "backup" is needed. I have personally added this requirement in the per-site FAQs of the sites I moderate, such as Chinese or Russian, stating this:
We do not forbid translation requests but note that this is not a translation service, so we will close any translation request that does not show research effort. Make sure you clearly show it in your question.
Perhaps it could use some improvement (I see a repeated expression) but the meaning is clear: we help anyone, no problem. But we're going to help, not do others' work. I think it's a matter of respect. Even Movies SE has a similar excerpt:
[...] Questions lacking in research, i.e. if a question is easily answered from an Actor's name on IMDB or Wikipedia will be closed.
But beyond that, I think this rule should be made official and I can't think of a site that wouldn't require it (even if not always). A starting wording I suggest:
When applicable, questions should always provide evident prior research effort. This way the community can answer better and more precisely your question basing on your previous results.
Or something similar, just an idea.