Grothendieck construction

Dear All, Where does the Grothendieck construction come from ? What is the original reference ? Here is the construction. Take a functor H:I-->Cat (the category of small categories) The objects are the pairs (i,a) where a is an object of H(i). A morphism (i,a)-->(j,b) consists of a morphism f:i-->j of I and a morphism H(f)(a)-->b of H(j). pg.