29 C.F.R. § 1630.13

Prohibited medical examinations and inquiries.

            (a)
            Pre-employment examination or inquiry.
             Except as permitted by § 1630.14, it is unlawful for a covered entity to conduct a medical examination of an applicant or to make inquiries as to whether an applicant is an individual with a disability or as to the nature or severity of such disability.

            (b)
            Examination or inquiry of employees.
             Except as permitted by § 1630.14, it is unlawful for a covered entity to require a medical examination of an employee or to make inquiries as to whether an employee is an individual with a disability or as to the nature or severity of such disability.

Official source