A fiber laser is stabilized by introducing a calcium fluoride (CaF$_2$) whispering-gallery-mode resonator as a filtering element in a ring cavity. It is set up using a semiconductor optical amplifier as a gain medium. The resonator is critically coupled through prisms, and used as a filtering element to suppress the laser linewidth. A three-cornered-hat method is used and shows a stability of 10−11 after 10μs. Using the self-heterodyne beat technique, the linewidth is determined to be 13kHz. This implies an enhancement factor of 10$^3$ with respect to the passive cavity linewidth.