Which muscle is tested in Froment's sign?

FROMENT SIGN: With weakness of the ulnar nerve innervated adductor pollicis muscle, a positive Froment sign may be observed. As the patient tries to hold a piece of paper placed between the thumb and the index finger, the thumb interphalangeal joint flexes in an attempt to substitute flexor pollicis longus activity for inadequate adductor pollicis strength. Ref: Bednar M.S., Light T.R. (2006). Chapter 10. Hand Surgery. In H.B. Skinner (Ed), CURRENT Diagnosis & Treatment in Ohopedics, 4e.