Quick search by citation:

5 U.S. Code § 8454 - Medical examination

An annuitant receiving a disability retirement annuity from the Fund shall be examined under the direction of the Office—
at the end of 1 year from the date of the disability retirement; and
annually thereafter until becoming 60 years of age;
unless the disability is permanent in character. If the annuitant fails to submit to examination as required by this section, payment of the annuity shall be suspended until continuance of the disability is satisfactorily established.