A.T. Robertson Commentary John 2:18

"The Jews therefore answered and said unto him, What sign showest thou unto us, seeing that thou doest these things?" — John 2:18 (ASV)

What sign shewest thou unto us? (Τ σημειον δεικνυεις ημιν; ). They may have heard of the "sign" at Cana or not, but they have rallied a bit on the outside of the temple area and demand proof for his Messianic assumption of authority over the temple worship. These traders had paid the Sadducees and Pharisees in the Sanhedrin for the concession as traffickers which they enjoyed. They were within their technical rights in this question.