  We report the first precision measurement of the absolute frequency of
$5s5p^{3}$P$_{0}\to5s6d^{3}$D$_{1}$ for all four stable Sr isotopes with an
accuracy of $\sim$25 kHz employing repumping induced spectroscopy. By combining
the isotope shifts of this transition with the existing measurement data on the
intercombination line, the King plot is established which reveals a deviation
from the linearity at the 5.2$\sigma$ level.
