It is in the AIM:
4−3−20. Exiting the Runway After Landing
b. Taxi clear of the runway unless otherwise
directed by ATC. An aircraft is considered clear of the
runway when all parts of the aircraft are past the
runway edge and there are no restrictions to its
continued movement beyond the runway holding
position markings. In the absence of ATC instructions,
the pilot is expected to taxi clear of the landing
runway by taxiing beyond the runway holding
position markings associated with the landing
runway, even if that requires the aircraft to protrude
into or cross another taxiway or ramp area. Once all
parts of the aircraft have crossed the runway holding
position markings, the pilot must hold unless further
instructions have been issued by ATC.
http://www.faa.gov/air_traffic/publi.../media/aim.pdf