From CFRs Title 14 Part 61
§ 61.3 Requirement for certificates, ratings, and authorizations.
(a) Pilot certificate. No person may serve as a required pilot flight crewmember of a civil aircraft of the United States, unless that person—
(1) Has a pilot certificate or special purpose pilot authorization issued under this part in that person's physical possession or readily accessible in the aircraft when exercising the privileges of that pilot certificate or authorization. However, when the aircraft is operated within a foreign country, a pilot license issued by that country may be used; and
(2) Has a photo identification that is in that person's physical possession or readily accessible in the aircraft when exercising the privileges of that pilot certificate or authorization. The photo identification must be a: [etc etc]