In the UK, it's an ANO which states something about people having to obey any lawful command issued for a variety of purposes. The ANOs are part of the UK's criminal statute, and breaking them can attract the full penalties of the law.
In the US, it's a bit different. You ask about the FARs. Well, there's obviously FAR 91.3(a) and to some extent parts of FAR 121.533 et seq, but remember that the FARs are Administrative Law and thus have both limited scope for penalities (eg. no option for jail) and limited applicability in respect of pax. There have, therefore, to be specific federal or state codes which address particular issues, such as 49 USC 46504 re interferring with aircrew.
Now, whether the comment I referred to about ordering people to begin singing songs would really stand up in court, I don't know, but the point that poster was making remains ie. that, in UK law at least, the test is 'lawful' not 'reasonable', and that the court will be loath to sustitute its judgement for that of the pilot. All this bleating about whether the captain's request was reasonable is thus pointless. If the captian asks you to do something lawful, do it. End of story. You are not in a position to second-guess his decision, and the court won't, either.