In this letter we describe how we use stellar dynamics information to constrain the shape of the stellar IMF in a sample of 27 early-type galaxies from the CALIFA survey. We obtain dynamical and stellar mass-to-light ratios, $ϒ_\mathrm{dyn}$ and $ϒ_{*}$, over a homogenous aperture of 0.5~$R_{e}$. We use the constraint $ϒ_\mathrm{dyn} \ge ϒ_{*}$ to test two IMF shapes within the framework of the extended MILES stellar population models. We rule out a single power law IMF shape for 75% of the galaxies in our sample. Conversely, we find that a double power law IMF shape with a varying high-mass end slope is compatible (within 1$σ$) with 95% of the galaxies. We also show that dynamical and stellar IMF mismatch factors give consistent results for the systematic variation of the IMF in these galaxies. ; Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letters