14 C.F.R. § 27.853
For each compartment to be used by the crew or passengers—
(c) If smoking is to be prohibited, there must be a placard so stating, and if smoking is to be allowed—
(2) Where the crew compartment is separated from the passenger compartment, there must be at least one illuminated sign (using either letters or symbols) notifying all passengers when smoking is prohibited. Signs which notify when smoking is prohibited must—
[Amdt. 27-17, 45 FR 7755, Feb. 4, 1980, as amended by Amdt. 27-37, 64 FR 45095, Aug. 18, 1999]