  Recent measurements of the doping dependence of the London penetration depth
\lambda(x) at low temperatures in clean samples of isovalent
BaFe_2[As_(1-x)P_x]_2 at T<<Tc [Hashimoto et al., Science 336, 1554 (2012)]
revealed a peak in \lambda(x) near optimal doping x=0.3. The observation of the
peak at T<<Tc, points to the existence of the quantum critical point (QCP)
beneath the superconducting dome. We associate such a QCP with the onset of a
spin- density-wave order and show that the renormalization of \lambda(x) by
critical magnetic fluctuations, gives rise to the observed feature. We argue
that the case of pnictides is conceptually different from a one-component
Galilean invariant Fermi liquid, for which correlation effects do not cause the
renormalization of the London penetration depth at T=0.
