You will not find this situation explicitly covered in the TDA rules
and Robert's Rules of Poker has this to say about The Showdown
'A player must show all cards in the hand face-up on the table to win any part of the pot.'
Now the technicalities are that once you call the bet, you have crossed the threshold into 'The Showdown'.
So what to do in this situation has 2 possibilities, each of them on equal standing depending on what 'rule' a TD wishes to choose to use. You cannot fault a TD for ruling it either way.