Is Rayleigh Quotient guaranteed to monotonically decrease in Conjugate Gradient Algorithm?

Question

I am using Linear Conjugate Gradient to solve for $x$ in $Ax=b$, where $A$ is PSD. I noticed experimentally that the Rayleigh Quotient $\frac{x_k^TAx_k}{x^Tx}$ of the accumulated solution $x_k$ is always monotonically decreasing with more iterations. This obsevation seems nontrivial to me and I wonder if it is theoretically guaranteed that this is true.