# Comoving Hubble radius in terms of expansion scale factor

I am currently reading Baumann's Cambridge lecture slides on cosmology and am confused on the notion of the Hubble radius. On page 10 it is stated that for a perfect fluid with constant equation of state $$w = P/\rho$$, the comoving Hubble radius is $$(aH)^{-1} \propto a^{0.5(1+3w)}$$, but I haven't the faintest idea where this equation came from, as no derivation is given. Can anyone provide one?

In Planck units, $$M_P=(8\pi G)^{-1}=1$$.