At Caesarea, what did a visiting prophet use to bind his own hands and feet as a sign about Paul?

Answer: Paul's belt

Agabus took Paul's belt (KJV: girdle), bound himself and said the Jews of Jerusalem would bind its owner and hand him to the Gentiles.

“Coming over to us, he took Paul’s belt, bound his own feet and hands, and said, “The Holy Spirit says: ‘In this way the Jews of Jerusalem will bind the owner of this belt and hand him over to the Gentiles.’””— Acts 21:11 (BSB)