Cosmic microwave background observations are used to constrain reheating to standard model (SM) particles after a period of inflation. As a light spectator field, the SM Higgs boson acquires large field values from its quantum fluctuations during inflation, gives masses to SM particles that vary from one Hubble patch to another, and thereby produces large density fluctuations. We consider both perturbative and resonant decay of the inflaton to SM particles. For the case of perturbative decay from coherent oscillations of the inflaton after high scale inflation, we find strong upper bounds on the reheat temperature for the inflaton decay into heavy SM particles. The strongest bounds arise in the case of reheating to top quarks where we find Treh less than or similar to Oo1012 thorn GeV for an inflaton mass of 1013 GeV. For the case of resonant particle production (preheating) to (Higgsed) SM gauge bosons, we find temperature fluctuations larger than observed in the cosmic microwave background for a range of gauge coupling that includes those found in the SM and conclude that such preheating cannot be the main source of reheating the Universe after inflation.