In the paper a unique phenomenon - the resonant generation of ultra-low frequency (ULF) wave by energetic electrons in the magnetosphere - is presented. On 27 October 2012 in the morning side of the magnetosphere Van Allen Probe A registered an ULF wave with period of 100 s and an amplitude of 0.7 nT. A cloud of energetic electrons was observed simultaneously with the wave. It is established that the electron cloud was injected into the magnetosphere as a result of a substorm. Electron fluxes in several energy channels were modulated with the frequency of the observed wave. It is shown that these flux modulations are caused by the drift resonance of 38 keV electrons and the wave being a fundamental harmonic of an Alfven mode with an azimuthal wavenumber m~ 110-115 propagating to the east, and generated through the gradient instability due to steep earthward density gradient of the resonant electrons.