  We report ultra-stable locking of a commercially available extended cavity
diode laser to a vibration-insensitive high finesse Fabry-Perot cavity. A servo
bandwidth of 2 MHz is demonstrated. The absolute stability of the diode laser
after locking is measured with a three-cornered-hat method. The resulting Allan
deviation reaches a level of $2.95\times10^{-15}$ at 1 s, corresponding to only
0.93 Hz linewidth, even without vibration isolation of the reference cavity.
