  We combine information from the clustering of HI galaxies in the 100% data
release of the Arecibo Legacy Fast ALFA survey (ALFALFA), and from the HI
content of optically-selected galaxy groups found in the Sloan Digital Sky
Survey (SDSS) to constrain the relation between halo mass $M_h$ and its average
total HI mass content $M_{\rm HI}$. We model the abundance and clustering of
neutral hydrogen through a halo-model-based approach, parametrizing the $M_{\rm
HI}(M_h)$ relation as a power law with an exponential mass cutoff. To break the
degeneracy between the amplitude and low-mass cutoff of the $M_{\rm HI}(M_h)$
relation, we also include a recent measurement of the cosmic HI abundance from
the $\alpha$.100 sample. We find that all datasets are consistent with a
power-law index $\alpha=0.44\pm 0.08$ and a cutoff halo mass $\log_{10}M_{\rm
min}/(h^{-1}M_\odot)=11.27^{+0.24}_{-0.30}$. We compare these results with
predictions from state-of-the-art magneto-hydrodynamical simulations, and find
both to be in good qualitative agreement, although the data favours a
significantly larger cutoff mass that is consistent with the higher cosmic HI
abundance found in simulations. Both data and simulations seem to predict a
similar value for the HI bias ($b_{\rm HI}=0.875\pm0.022$) and shot-noise power
($P_{\rm SN}=92^{+20}_{-18}\,[h^{-1}{\rm Mpc}]^3$) at redshift $z=0$.
