CFL condition in Higher Dimension

Hello everyone!
I've just one question. I know that for 2D wave equation CFL condition
is k<=(h/sqrt(2)) and for 3D wave equation CFL condition is
k<=(h/sqrt(3)) where k is a time step and h is a spatial step. But
where can I find proofs of those conditions? Maybe in some papers or
books? Any help would be very apprecieted.
                                                John Maly