  We report the first measurement of $x_B$-scaling in $(e,e'p)$ cross-section
ratios off nuclei relative to deuterium at large missing-momentum of $350 \leq
p_{miss} \leq 600$ MeV/c. The observed scaling extends over a kinematic range
of $0.7 \leq x_B \leq 1.8$, which is significantly wider than $1.4 \leq x_B
\leq 1.8$ previously observed for inclusive $(e,e')$ cross-section ratios. The
$x_B$-integrated cross-section ratios become constant (i.e., scale) beginning
at $p_{miss}\approx k_F$, the nuclear Fermi momentum. Comparing with
theoretical calculations we find good agreement with Generalized Contact
Formalism calculations for high missing-momentum ($> 375$ MeV/c), suggesting
the observed scaling results from interacting with nucleons in short-range
correlated (SRC) pairs. For low missing-momenta, mean-field calculations show
good agreement with the data for $p_{miss}\le k_F$, and suggest that
contributions to the measured cross-section ratios from scattering off single,
un-correlated, nucleons are non-negligible up to $p_{miss}\approx 350$ MeV/c.
Therefore, SRCs become dominant in nuclei at $p_{miss}\approx 350$ MeV/c, well
above the nuclear Fermi Surface of $k_F \approx 250$ MeV/c.
