14 CFR §125.261

1. (a)No certificate holder may use any person as an airman nor may any person serve as an airman unless that person—
   1. (1)Holds an appropriate current airman certificate issued by the FAA;
   2. (2)Has any required appropriate current airman and medical certificates in that person's possession while engaged in operations under this part; and
   3. (3)Is otherwise qualified for the operation for which that person is to be used.
2. (b)Each airman covered by paragraph (a) of this section shall present the certificates for inspection upon the request of the Administrator.