we know that
[volume of a rectangular prism]=[area of the base]*h
and 
[volume of a pyramid]=[area of the base]*h/3
if both have congruent bases and equal heights
so
 [volume of a pyramid]=(1/3)*[volume of a rectangular prism]
 The statement is 
 [volume of a pyramid]=(1/3)*[volume of a rectangular prism]