  Deeply bound 1s states of $\pi^-$ in $^{115,119,123}$Sn were preferentially
observed using the Sn($d$,$^3$He) pion-transfer reaction under the recoil-free
condition. The 1s binding energies and widths were precisely determined, and
were used to deduce the isovector parameter of the s-wave pion-nucleus
potential to be $b_1 =-0.115\pm 0.007 ~m_{\pi}^{-1}$. The observed enhancement
of $|b_1|$ over the free $\pi N$ value ($b_1^{\rm free}/b_1 = 0.78 \pm 0.05$)
indicates a reduction of the chiral order parameter, $f^{*}_{\pi}
(\rho)^2/f_{\pi}^2 \approx 0.64$, at the normal nuclear density, $\rho =
\rho_0$.
