How fast, and how foreseeable, is magma ascent is one of the most compelling and unanswered issues of volcanology. The velocity of the magma upwelling depends on the local conditions of the volcanic conduit and rheology of the magma. During magma emplacement in the shallow crust, transient variations of physical properties underneath active volcanoes are expected and in a few cases observed. The predictability of such changes strongly depends on how fast this process is, compared to our ability to handle geophysical data and consistently resolve transient anomalies in the physical properties of the medium. Mount Etna (Italy) is a perfect natural laboratory to investigate such issues, due to the almost continuous magmatic activity and the high quality of seismologic and geodetic data. Here we show, for the first time, that seismic attenuation of local earthquakes strongly increases due to the emplacement of magma within the crust, forecasting an incipient eruption at Mount Etna.