  A central question in a large class of strongly correlated electron systems,
including heavy fermion compounds and iron pnictides, is the identification of
different phases and their origins. It has been shown that the
antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by
Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and
that the competition between this interaction and Kondo effect is responsible
for quantum criticality. However, conclusive experimental evidence of the RKKY
interaction in pnictides is lacking. Here, using high resolution $^{23}$Na NMR
measurements on lightly Cu-doped metallic single crystals of
NaFe$_{1-x}$Cu$_{x}$As ~($x \approx 0.01$) and numerical simulation, we show
direct evidence of the RKKY interaction in this pnictide system. Aided by
computer simulation, we identify the $^{23}$Na NMR satellite resonances with
the RKKY oscillations of spin polarization at Fe sites. Our $^{23}$Na
spin-lattice and spin-spin relaxation data exhibits signature of an itinerant
and inhomogeneous AFM phase in this system, accompanied by a simulation of
Cu-induced perturbation to the ordered moments on the Fe sites. Our NMR results
indicate coexistence of local and itinerant magnetism in lightly Cu-doped
NaFe$_{1-x}$Cu$_{x}$As.
