Upper bound of Sine Kernel

Question

How can I show that the upper bound (spectral radius) of the Sine kernel is 1, i.e. $|\lambda_{max}|\le1$? $$K(x,y)=\frac{\text{sin}\{\pi(x-y)\}}{\pi(x-y)}$$ The corresponding integral equation is, $$\lambda\psi(x)=\int_{-s/2}^{s/2}K(x,y)\psi (y)dy$$