Natural convection is studied in a square cavity filled with a hybrid nanofluid in the presence of a horizontal magnetic field. Cooling temperatures are applied to the left and right sides of the cavity, while the other walls are adiabatic. Inside the cavity, four uniformly distributed square blocks generate heat flux. The finite volume method, using the SIMPLE algorithm and considering the Boussinesq approximation, is employed for the analysis. The results are presented in terms of streamlines, isotherms and profiles of the Nusselt number, Hartmann number, Rayleigh number and volume fraction. The results indicate that an increase in Rayleigh number and volume fraction improves heat transfer within the cavity. Conversely, the application of the magnetic field reduces heat transfer efficiency.