47 CFR §87.103

1. (a)Stations at fixed locations. The license or a photocopy must be posted or retained in the station's permanent records.
2. (b)Aircraft radio stations. The license must be either posted in the aircraft or kept with the aircraft registration certificate. If a single authorization covers a fleet of aircraft, a copy of the license must be either posted in each aircraft or kept with each aircraft registration certificate.
3. (c)Aeronautical mobile stations. The license must be retained as a permanent part of the station records.