We report on the development and characterization of a passive heterodyne hot electron bolometer (HEB) imager operating at 850 GHz. HEB detectors provide unprecedented sensitivity and spectral resolution at terahertz frequencies covering the frequency range of 0.5-5 THz. Terahertz imagers based on HEB technology have sufficient sensitivity to operate in a passive imaging mode, thus eliminating the need for active illumination. We demonstrated a fully automated passive imaging system based on our HEB technology. The front-end heterodyne detector includes a quasi-optically coupled HEB device in close proximity to a monolithic microwave integrated circuit low-noise IF amplifier. We integrated a compact commercial harmonic multiplier local oscillator (LO) source to provide the LO biasing for the HEB detector. Our high spectral resolution terahertz imager has a noise equivalent temperature difference value of 0.4 K and a spatial resolution of approximately 4 mm.