14 C.F.R. § 121.555 – Compliance with approved routes and limitations: Domestic and flag operations.

No pilot may operate an airplane in scheduled air transportation—

(a) Over any route or route segment unless it is specified in the certificate holder's operations specifications; or
(b) Other than in accordance with the limitations in the operations specifications.

[Docket 6258, 29 FR 19219, Dec. 31, 1964, as amended by Amdt. 121-253, 61 FR 2614, Jan. 26, 1996]