Pairwise smoothScatter with correlation on upper diagonal in R

Question

Can someone explain to me how to correctly write the panel.cor function to get the labels for pairwise correlations of the columns of the matrix (my_matrix) supplied to the pairs function?

par(mfrow=c(1,1))
pairs(my_matrix,
      upper.panel = panel.cor, 
      panel = function(...) smoothScatter(..., nbin=64,
                                            nrpoints = 0,  
                                            add = TRUE), gap = 0.2, ylim = c(-5,5), xlim=c(-5,5)
      )   

panel.cor <- function(x, y, digits=3, prefix="",  ...) {
  r <- cor(x,y)
}

The function supplied to the panel argument is from the examples section of the smoothScatter help page.