  We study an inverse source scattering problem for the Schr\"odinger equation
with a quadratic nonlinearity. In general, uniqueness of inverse source
problems can not be guaranteed at a fixed energy. Therefore, additional
information is required for the source in order to obtain a unique solution. By
adding reference point sources, we show that a general source function could be
uniquely determined from boundary measurements at a fixed wavenumber. This
method does not apply to inverse source problems of linear equations since it
uses the nonlinearity as a tool. The proof utilizes the method of linearization
to reduce the nonlinear inverse source scattering problem to the inverse
potential scattering problem of the linear Schr\"odinger equation.
