Danilo,

A good source is the 1995 book "The natural rate of unemployment -
reflections on 25 years of the hypothesis", ed. by R. Cross (CUP),
especially the chapter by Phelps.

But I agree with Pat that the natural rate hypothesis (if not the
word) is much older than that. See e.g. my paper with John Presley on
"The Robertson connection between the natural rates of interest and
unemployment", in Structural Change and Economic Dynamics 20 (June):
136-150, 2009.

Mauro Boianovsky