Aims: We report the detection of transit timing variations (TTVs) well in excess of one hour in the Kepler multi-planet candidate system KOI 806. This system exhibits transits consistent with three separate planets - a Super-Earth, a Jupiter, and a Saturn - lying very nearly in a 1:2:5 resonance, respectively. Methods: We used the Kepler public data archive and observations with the Gran Telescopio de Canarias to compile the necessary photometry. Results: For the largest candidate planet (KOI 806.02) in this system, we detected a sizable transit timing variation of -103.5 ± 6.9 min against previously published ephemeris. We did not obtain a strong detection of a transit color signature consistent with a planet-sized object; however, we did not detect a color difference in transit depth, either. Conclusions: The substantial TTV is consistent with theoretical predictions that exoplanets in resonance can produce strong transit timing variations, particularly if the orbits are eccentric. The presence of such TTVs among the bodies in this systems indicates that KOI 806 is very likely to be a planetary system. This is supported by the lack of a strong color dependence in the transit depth, which would suggest a blended eclipsing binary. Photometric data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/536/L9